Update SparseDtype user guide doc
Tiny doc nit. The doc says you can pass both dtype and fill_value to SparseDtype then only passes one in the example.
The user guide already shows an example on the next line of passing a fill value though - perhaps, instead of making both examples pass a fill value, the wording could be changed so that the first example shows construction with just the dtype of the non-sparse values. Something like

```
A :class:`SparseDtype` may be constructed by passing the former

.. ipython:: python
   pd.SparseDtype(np.dtype('datetime64[ns]'))

, in which case the default fill value for a given NumPy dtype is the "missing" value for that dtype. Note that it may be overridden by passing a `fill_value`, for example

.. ipython:: python
   pd.SparseDtype(np.dtype('datetime64[ns]'),
                  fill_value=pd.Timestamp('2017-01-01'))
``` Good point @MarcoGorelli. Actually I changed some other wording as well (it's not entirely true that a missing value indicator will be used for numpy dtypes, e.g., not for integers)

```python
In [1]: import numpy as np                                                                                                                                                                                   

In [2]: import pandas as pd                                                                                                                                                                                  

In [3]: pd.SparseDtype(np.int64)                                                                                                                                                                             
Out[3]: Sparse[int64, 0]
``` Thanks @dsaxton. The CI failure was unrelated

```
Skipping uploading of coverage data.

=================================== ERRORS ====================================
_____ ERROR at setup of TestParquetPyArrow.test_s3_roundtrip_explicit_fs ______
```