DOC: GroupBy.head()/tail() documentation
- [x] closes #6721
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Update DOC to make it consistent with actual behavior
added double backticks to `as_index` as suggested Hello @mtrbean! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-08-20 17:29:07 UTC @jorisvandenbossche I've updated the PR per your comment @jorisvandenbossche @datapythonista ping again