Fix broken link in docs of DataFrame.to_hdf
The link in https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_hdf.html in "See also" seems to be broken. This should fix it.

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

thanks @felixdivo 