Fix docstring validation errors in pandas.Index.slice_indexer
- [ ] closes https://github.com/pandanistas/pandanistas_sprint_jakarta2020/issues/6
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@giovanism can you have a look at the last comments? Closing as stale, @giovanism let is know if you want to finish this.