Upgrade pygrep, use in-built rst-directive-colons
I got a PR in to pre-commit/pygrep-hooks to make the "incorrect-sphinx-directives" hook a built-in one (as it's not specific to pandas) :tada: So, let's use it?

They also have another useful hook (`rst-inline-touching-normal`), which caught a couple of errors which I've fixed below.

The reason why I've kept `rst-backticks` to just `rst` files (as opposed to `types: [text]` like the other two) is that in docstrings, the numpy guide instructs to use single backticks for variable (though if it was ever desirable to change that, then this hook would make it pretty easy to check for consistency)