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. :)
```

