BUG&TST: HTML formatting error in Styler.render() in rowspan attribute
- [x] closes [#38234](https://github.com/pandas-dev/pandas/issues/38234)
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
