DOC: Fix EX02 in pandas.Index.get_loc
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Related to #27977. 

```
################################################################################
################################## Validation ##################################
################################################################################

5 Errors found:
        No extended summary found
        Parameter "key" has no description
        The first line of the Returns section should contain only the type, unless multiple values are being returned
        Return value has no description
        See Also section not found
```