DOC: Fix SA04 errors from DataFrame.melt
- [ ] closes https://github.com/pandanistas/pandanistas_sprint_jakarta2020/issues/10
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@giovanism Thank you for the PR!

Can you run "black" on ```pandas/core/frame.py```? Closing as stale. @giovanism in case you want to further work on this (based on your gitter comment), you might need to open a new PR. 
(I can't reopen this PR, github indicates this is because there has been a force push to this branch after the PR was closed)