Added missing import in boxplot_frame_groupby example
- [X] closes #28903 
- [x] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Thanks @gsyqax - did you build this page of the documentation locally to check it works? @MarcoGorelli yes, it is working.

I noticed the example also makes use of 'pd' and 'np' alias without importing and aliasing pandas and numpy. I don't think this can lead to confusion. Do you think I should also add those imports? That's OK, it fits with the [docstring guide](https://pandas.pydata.org/docs/development/contributing_docstring.html)

> Import required libraries (except numpy and pandas) > @MarcoGorelli yes, it is working.

I just tried building it with your changes and I don't see any plot though, do you?

See the guide I linked to above for how to display plots in documentation examples I'm not seeing any plots either. I thought a plot was not needed since the current version is not showing any either.

I will add the plots and commit again Hello @gsyqax! 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 2020-07-06 14:53:37 UTC @MarcoGorelli I added some plots and made a few minor changes to make everything clearer, hope it's OK. Also removed whitespace as indicated by pep8speaks. Also, I don't think a whatsnew note is necessary for documentation improvements, could you please revert that change? Great! Many thanks @MarcoGorelli for all the help, really appreciate it! cc @datapythonista @jorisvandenbossche @WillAyd  try `boxplot_frame_groupby(grouped, rot=45, fontsize=12, figsize=(8, 10))` for instance? @gsyqax 

it will avoid having overlapped subplots and make doc look better  Slight preference for what @jorisvandenbossche has suggested above but not a blocker so OK either way I applied the change to the plot fig size and replaced boxplot_frame_groupby by df.groupby.boxplot. I'm keeping the intermediate var 'grouped' to make things easier to read lgtm. @TomAugspurger  Thanks @gsyqax!