fix: make rule message punctuation more consistent and address some typos

[G-Rath]

I noticed a spelling mistake in valid-expect that I'd fixed in eslint-plugin-jest which revealed jest-community/eslint-plugin-jest#1444 has not been ported over, so this mostly does that though also has a few other adjustments.

There's some more improvements that could be made which I don't mind doing (namely around the use of code quotes), but didn't want to do too much without first confirming folks are happy with this kind of "format" 🙂

[veritem]

All of these look good to me! Feel free to open another pr if there is anything else you'd like to improve.