DOC: Update pandas.DataFrame.droplevel docstring
- [ ] closes https://github.com/pandanistas/pandanistas_sprint_jakarta2020/issues/23
- [ ] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Output of `python scripts/validate_docstrings.py pandas.DataFrame.droplevel`:
```
################################################################################
#################### Docstring (pandas.DataFrame.droplevel) ####################
################################################################################

Return DataFrame with requested index / column level(s) removed.

.. versionadded:: 0.24.0

Parameters
----------
level : int, str, or list-like
    If a string is given, must be the name of a level
    If list-like, elements must be names or positional indexes
    of levels.

axis : {0 or 'index', 1 or 'columns'}, default 0
    Axis along which the level(s) is removed:

    * 0 or 'index': remove level(s) in column.
    * 1 or 'columns': remove level(s) in row.

Returns
-------
DataFrame
    DataFrame with requested index / column level(s) removed.

Examples
--------
>>> df = pd.DataFrame([
...     [1, 2, 3, 4],
...     [5, 6, 7, 8],
...     [9, 10, 11, 12]
... ]).set_index([0, 1]).rename_axis(['a', 'b'])

>>> df.columns = pd.MultiIndex.from_tuples([
...     ('c', 'e'), ('d', 'f')
... ], names=['level_1', 'level_2'])

>>> df
level_1   c   d
level_2   e   f
a b
1 2      3   4
5 6      7   8
9 10    11  12

>>> df.droplevel('a')
level_1   c   d
level_2   e   f
b
2        3   4
6        7   8
10      11  12

>>> df.droplevel('level_2', axis=1)
level_1   c   d
a b
1 2      3   4
5 6      7   8
9 10    11  12

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

1 Errors found:
	See Also section not found
```