DOC: use the commonly accepted capitalization of LaTeX
Latex -> LaTeX

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

look like heading have a special capitalization rule:
doc/source/reference/io.rst:102:Heading capitalization formatted incorrectly. Please correctly capitalize "LaTeX" to "Latex"