DOC: update the Index.get_level_values 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.Index.get_level_values)  ##################
################################################################################

Return an Index of values for requested level.

Return an Index of values for requested level, equal to the length
of the index.

Parameters
----------
level : int or str
    It is either the integer position of the level in the
    MultiIndex, or the `name` of the level.
    If given as an integer, it must be between 0 and the
    number of levels.

Returns
-------
values : Index
    `self`, as there is only one level in the Index.

See also
---------
pandas.MultiIndex.get_level_values : get values for a level of a
                                     MultiIndex

Notes
---------
For `Index`, level should be 0, since there are no multiple levels.

Examples
---------

Create an Index:

>>> idx = pd.Index(list('abc'))

Get level value by supplying level as integer:

>>> idx.get_level_values(0)
Index(['a', 'b', 'c'], dtype='object')

Create a MultiIndex:

>>> mi = pd.MultiIndex.from_arrays((list('abc'), list('def')))
>>> mi.names = ['level_1', 'level_2']

Get level values by supplying level as name:

>>> mi.get_level_values('level_2')
Index(['d', 'e', 'f'], dtype='object', name='level_2')

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

Docstring for "pandas.Index.get_level_values" correct. :)
```