Update en-US.md

Updates made due to source file revision by Jeff Garver per bug https://bugzilla.mozilla.org/show_bug.cgi?id=1058303, comment #11.
It was the only one I saw... Not sure how easy it'd be to run a spellchecker against __ number of en-us.md files though (as I'm guessing running a spell checker against a ja.md file would result in more false positives than actual typos).

I could probably write a quick Node.js script which concatenates a bunch of separate language files into a single file which somebody could open in Word or Pages or something else with a spellchecker or dictionary (if anybody is interested).