DOC: fix DataFrame.plot docs 
- [x] fixes #29489 
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
 
@lithomas1 can you post a screenshot of how this looks after building? ![image](https://user-images.githubusercontent.com/47963215/72863198-9ae0e100-3c84-11ea-8afc-d645b3af1435.png)
cc @WillAyd 
Should I add tests for this too in order to prevent regression in the future? Thanks. Let's merge this and see how they look when built.