DOC: Fix doc for DataFrame.plot subplots argument
`.. versionadded` was missing a newline between it and the previous paragraph, and so wasn't being rendered.

- [x] ~closes #xxxx (Replace xxxx with the GitHub issue number)~ n/a
- [x] ~[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~ n/a
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] ~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~ n/a
- [x] ~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~ n/a

<details>
<summary>Output from <code>validate_docstrings.py</code></summary>

```

################################################################################
###################### Docstring (pandas.DataFrame.plot)  ######################
################################################################################

Make plots of Series or DataFrame.

Uses the backend specified by the
option ``plotting.backend``. By default, matplotlib is used.

Parameters
----------
data : Series or DataFrame
    The object for which the method is called.
x : label or position, default None
    Only used if data is a DataFrame.
y : label, position or list of label, positions, default None
    Allows plotting of one column versus another. Only used if data is a
    DataFrame.
kind : str
    The kind of plot to produce:

    - 'line' : line plot (default)
    - 'bar' : vertical bar plot
    - 'barh' : horizontal bar plot
    - 'hist' : histogram
    - 'box' : boxplot
    - 'kde' : Kernel Density Estimation plot
    - 'density' : same as 'kde'
    - 'area' : area plot
    - 'pie' : pie plot
    - 'scatter' : scatter plot (DataFrame only)
    - 'hexbin' : hexbin plot (DataFrame only)
ax : matplotlib axes object, default None
    An axes of the current figure.
subplots : bool or sequence of iterables, default False
    Whether to group columns into subplots:

    - ``False`` : No subplots will be used
    - ``True`` : Make separate subplots for each column.
    - sequence of iterables of column labels: Create a subplot for each
      group of columns. For example `[('a', 'c'), ('b', 'd')]` will
      create 2 subplots: one with columns 'a' and 'c', and one
      with columns 'b' and 'd'. Remaining columns that aren't specified
      will be plotted in additional subplots (one per column).

      .. versionadded:: 1.5.0

sharex : bool, default True if ax is None else False
    In case ``subplots=True``, share x axis and set some x axis labels
    to invisible; defaults to True if ax is None otherwise False if
    an ax is passed in; Be aware, that passing in both an ax and
    ``sharex=True`` will alter all x axis labels for all axis in a figure.
sharey : bool, default False
    In case ``subplots=True``, share y axis and set some y axis labels to invisible.
layout : tuple, optional
    (rows, columns) for the layout of subplots.
figsize : a tuple (width, height) in inches
    Size of a figure object.
use_index : bool, default True
    Use index as ticks for x axis.
title : str or list
    Title to use for the plot. If a string is passed, print the string
    at the top of the figure. If a list is passed and `subplots` is
    True, print each item in the list above the corresponding subplot.
grid : bool, default None (matlab style default)
    Axis grid lines.
legend : bool or {'reverse'}
    Place legend on axis subplots.
style : list or dict
    The matplotlib line style per column.
logx : bool or 'sym', default False
    Use log scaling or symlog scaling on x axis.

logy : bool or 'sym' default False
    Use log scaling or symlog scaling on y axis.

loglog : bool or 'sym', default False
    Use log scaling or symlog scaling on both x and y axes.

xticks : sequence
    Values to use for the xticks.
yticks : sequence
    Values to use for the yticks.
xlim : 2-tuple/list
    Set the x limits of the current axes.
ylim : 2-tuple/list
    Set the y limits of the current axes.
xlabel : label, optional
    Name to use for the xlabel on x-axis. Default uses index name as xlabel, or the
    x-column name for planar plots.

    .. versionadded:: 1.1.0

    .. versionchanged:: 1.2.0

       Now applicable to planar plots (`scatter`, `hexbin`).

    .. versionchanged:: 2.0.0

        Now applicable to histograms.

ylabel : label, optional
    Name to use for the ylabel on y-axis. Default will show no ylabel, or the
    y-column name for planar plots.

    .. versionadded:: 1.1.0

    .. versionchanged:: 1.2.0

       Now applicable to planar plots (`scatter`, `hexbin`).

    .. versionchanged:: 2.0.0

        Now applicable to histograms.

rot : float, default None
    Rotation for ticks (xticks for vertical, yticks for horizontal
    plots).
fontsize : float, default None
    Font size for xticks and yticks.
colormap : str or matplotlib colormap object, default None
    Colormap to select colors from. If string, load colormap with that
    name from matplotlib.
colorbar : bool, optional
    If True, plot colorbar (only relevant for 'scatter' and 'hexbin'
    plots).
position : float
    Specify relative alignments for bar plot layout.
    From 0 (left/bottom-end) to 1 (right/top-end). Default is 0.5
    (center).
table : bool, Series or DataFrame, default False
    If True, draw a table using the data in the DataFrame and the data
    will be transposed to meet matplotlib's default layout.
    If a Series or DataFrame is passed, use passed data to draw a
    table.
yerr : DataFrame, Series, array-like, dict and str
    See :ref:`Plotting with Error Bars <visualization.errorbars>` for
    detail.
xerr : DataFrame, Series, array-like, dict and str
    Equivalent to yerr.
stacked : bool, default False in line and bar plots, and True in area plot
    If True, create stacked plot.
secondary_y : bool or sequence, default False
    Whether to plot on the secondary y-axis if a list/tuple, which
    columns to plot on secondary y-axis.
mark_right : bool, default True
    When using a secondary_y axis, automatically mark the column
    labels with "(right)" in the legend.
include_bool : bool, default is False
    If True, boolean values can be plotted.
backend : str, default None
    Backend to use instead of the backend specified in the option
    ``plotting.backend``. For instance, 'matplotlib'. Alternatively, to
    specify the ``plotting.backend`` for the whole session, set
    ``pd.options.plotting.backend``.

    .. versionadded:: 1.0.0

**kwargs
    Options to pass to matplotlib plotting method.

Returns
-------
:class:`matplotlib.axes.Axes` or numpy.ndarray of them
    If the backend is not the default matplotlib one, the return value
    will be the object returned by the backend.

Notes
-----
- See matplotlib documentation online for more on this subject
- If `kind` = 'bar' or 'barh', you can specify relative alignments
  for bar plot layout by `position` keyword.
  From 0 (left/bottom-end) to 1 (right/top-end). Default is 0.5
  (center)

################################################################################
################################## Validation ##################################
################################################################################

3 Errors found:
        Unknown parameters {'loglog', 'xlim', 'title', 'subplots', 'legend', 'use_index', 'layout', 'position', 'include_bool', 'sharex', 'rot', 'x', 'ylabel', 'yerr', 'style', 'table', 'figsize', 'grid', 'stacked', 'fontsize', 'sharey', 'yticks', 'secondary_y', 'logx', 'kind', 'logy', 'ax', 'xerr', 'colormap', 'y', 'colorbar', 'mark_right', 'ylim', '**kwargs', 'backend', 'xlabel', 'xticks'}
        See Also section not found
        No examples section found
```

</details>
Thanks @otaithleigh 