Various typo fixes #4 - only in comments, no code changes.

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.

Probably just one more type-fix merge after this one, which will be the more-commonly updated files Marlin.h and Marlin_main.cpp - I thought it would be best to keep those separate.