Fix docstring for to_markdown buf parameter
- [ ] ~~closes #xxxx~~ (N/A)
- [ ] ~~tests added / passed~~ (N/A)
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] ~~whatsnew entry~~ (N/A)

thanks @jakevdp 