DOC: update the pandas.Series.str.endswith docstring

[dcreekp]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

• [ X] PR title is "DOC: update the 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]

Codecov Report

Merging #20491 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            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%)	⬇️
#single	41.88% <33.33%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/strings.py	98.32% <100%> (ø)	⬆️
pandas/plotting/_converter.py	65.07% <0%> (-1.74%)	⬇️

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[TomAugspurger]

Thanks @dcreekp