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.
Corneliu Rusu (HTP): Localized FMD section in cs-CZ.
Merge pull request #238 from CorneliuRusu/master

Corneliu Rusu (HTP): Localized FMD section in cs-CZ.
Update en-US.md

fixing a typo - Protogype -> Prototype.
Adding space

"usingyour" -> "using your"
Merge pull request #239 from pdehaan/patch-4

Adding space
Merge pull request #237 from peiying2/patch-73

Update en-US.md