DOC: update the pandas.Series.dt.is_quarter_end 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.dt.is_quarter_end)  #################
################################################################################

Return a boolean indicating whether the date is the last day of a
quarter.

Returns
-------
is_quarter_end : Series of boolean.

See Also
--------
quarter : Return the quarter of the date.

is_quarter_start : Return a boolean indicating whether the date is the
    first day of the quarter.

Examples
--------
>>> df = pd.DataFrame({'dates': pd.date_range("2017-03-30",
...                   periods=4)})
>>> df.assign(quarter = df.dates.dt.quarter,
...          is_quarter_end = df.dates.dt.is_quarter_end)
       dates  quarter    is_quarter_end
0 2017-03-30        1             False
1 2017-03-31        1              True
2 2017-04-01        2             False
3 2017-04-02        2             False

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

Errors found:
	No summary found (a short summary in a single line should be present at the beginning of the docstring)

```

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.

* No extended summary required.
* Short summary exceeds one line by one word, hope that is ok
