DOC: Fix docstring for tsplot
Browsing through pandas, I noticed a small plotting function who's docs were incomplete / out of date. This is a small update to the docstring for tsplot in plotting/_timeseries. 

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry