Various typo fixes #3 - only in comments, no code changes.

This one is more likely to have conflicts - it's the LCD code that seems to get touched more often.

Low priority fixes really, but I've been making some minor spelling and grammar corrections to the codebase, and will be submitting them as pull-requests a few files at a time to reduce the chance of any merge conflicts.

I will try to keep my branch as close to Marlin_v1 as possible, but if there are any conflicts just kick this back to me.