DOC: update the docstring pandas.Series.dt.is_month_start
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.dt.is_month_start)  #################
################################################################################

Return a boolean indicating if the date is the first day of the month.

Returns
-------
is_month_start : Series of boolean

See Also
--------
is_month_end : Returns a boolean indicating if the date is the last day
               of the month.

Examples
--------
>>> dates = pd.Series(pd.date_range("2018-02-27", periods=3))
>>> dates
0   2018-02-27
1   2018-02-28
2   2018-03-01
dtype: datetime64[ns]
>>> dates.dt.is_month_start
0    False
1    False
2    True
dtype: bool

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

Errors found:
	No extended summary found

```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

Extended summary not required in this case (confirmed by peers).
Hello @janelle! Thanks for submitting the PR.

 - In the file [`pandas/core/indexes/datetimes.py`](https://github.com/pandas-dev/pandas/blob/232520fc6625005ec6cc0d86e7d979ee1adbdb94/pandas/core/indexes/datetimes.py), following are the PEP8 issues :

> [Line 1739:80](https://github.com/pandas-dev/pandas/blob/232520fc6625005ec6cc0d86e7d979ee1adbdb94/pandas/core/indexes/datetimes.py#L1739): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (93 > 79 characters)
> [Line 1768:80](https://github.com/pandas-dev/pandas/blob/232520fc6625005ec6cc0d86e7d979ee1adbdb94/pandas/core/indexes/datetimes.py#L1768): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (96 > 79 characters)

 Please, add "DOC: " prefix to the PR. This LGTM once you fix the line to long issues. Ping me once that's fixed and we'll merge it. closing as I might have mixed up two branches by accident, sorry
will open again for pandas.Series.dt.is_month_last to avoid confusion