DOC: update the pandas.Series.str.endswith docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [ X] PR title is "DOC: update the <your-function-or-method> docstring"
- [X ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [ X] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X ] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [ X ] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
#################### Docstring (pandas.Series.str.endswith) ####################
################################################################################

Test if the end of each string element matches a pattern.

Equivalent to :meth:`str.endswith`.

Parameters
----------
pat : str
    Character sequence. Regular expressions are not accepted.
na : object, default NaN
    Object shown if element tested is not a string.

Returns
-------
Series or Index of bool
    A Series of booleans indicating whether the given pattern matches
    the end of each string element.

See Also
--------
str.endswith : Python standard library string method.
Series.str.startswith : Same as endswith, but tests the start of string.
Series.str.contains : Tests if string element contains a pattern.

Examples
--------
>>> s = pd.Series(['bat', 'bear', 'caT', np.nan])
>>> s
0     bat
1    bear
2     caT
3     NaN
dtype: object

>>> s.str.endswith('t')
0     True
1    False
2    False
3      NaN
dtype: object

Specifying `na` to be `False` instead of `NaN`.

>>> s.str.endswith('t', na=False)
0     True
1    False
2    False
3    False
dtype: bool

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

Docstring for "pandas.Series.str.endswith" correct. :)
```


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=h1) Report
> Merging [#20491](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/402ad45da3f1c44da6594a773e5ba567ee1966a5?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20491/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20491      +/-   ##
==========================================
- Coverage   91.85%   91.82%   -0.03%     
==========================================
  Files         152      152              
  Lines       49235    49235              
==========================================
- Hits        45224    45212      -12     
- Misses       4011     4023      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.88% <33.33%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20491/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20491/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=footer). Last update [402ad45...6a5ff51](https://codecov.io/gh/pandas-dev/pandas/pull/20491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @dcreekp 