DOC: Fix groupby nth
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Current API doc looks incorrect. Fixed it to meet #11039. 
- http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.core.groupby.GroupBy.nth.html

cc @behzadnouri 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13810?src=pr) is 85.23% (diff: 100%)

> Merging [#13810](https://codecov.io/gh/pydata/pandas/pull/13810?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13810   diff @@
==========================================
  Files           140        140          
  Lines         50420      50420          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42974      42976     +2   
+ Misses         7446       7444     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [31f8e4d...948255b](https://codecov.io/gh/pydata/pandas/compare/31f8e4dc8af8f0d109f366d0b726aef210bf7904...948255b4835eae0eff850676aa6edc289f247ca6?src=pr)
 Can you show an example with `as_index=False` as well?
 @jorisvandenbossche good idea. added.
 thanks!
