DOC: minor tweaks to formatting on SQL comparison page
Adding code formatting, missing punctuation, etc. No changes of substance.

- [ ] ~~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~~
thanks @afeld 