CLN: w3 formatting
- [x] closes #35706
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Cleans a very minor formatting error to be consistent with w3schools specification.

`colspan=2   --->  colspan="2"`

Note this generally has no effect in browsers parsing HTML.
Thanks @attack68 - is it possible to write a test for this? seemed like overkill, but included.. thanks @attack68 

reason for the tests is to lock down the behavior; otherwise someone can accidently revert this and we wouldn't know :->