Fix documentation for new float_precision on read_csv
- [x] closes https://github.com/pandas-dev/pandas/pull/36228#discussion_r487587534
- [ ] tests added / passed
     N/A
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
     N/A

A followup to PR #36228 to get the documentation string right for version changed. (was in wrong place, missing blank line)

