DOC: update the Index.get_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_values)  #####################
################################################################################

Return `Index` data as an `ndarray`.

Because this is a wrapper function around Index.values, it is useful
when one explicitly wants to use a getter.

Returns
-------
numpy.ndarray
    A one-dimensional numpy `array` of the indexes.

See Also
--------
Index
    The basic object storing axis labels for all pandas objects.
Index.values
    The original function around which `get_values` creates a wrapper.

Examples
--------
Getting the `Index` values of a `DataFrame`:

>>> df = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]],
...                    index=['a', 'b', 'c'], columns=['A', 'B', 'C'])
>>> df
   A  B  C
a  1  2  3
b  4  5  6
c  7  8  9
>>> df.index.get_values()
array(['a', 'b', 'c'], dtype=object)

Standalone `Index` values:

>>> idx = pd.Index(['1', '2', '3'])
>>> idx.get_values()
array(['1', '2', '3'], dtype=object)

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

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

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.
