typo fixes - https://github.com/vlajos/misspell_fixer

Summary: Closes https://github.com/facebook/hhvm/pull/4179

Reviewed By: @JoelMarcey

Differential Revision: D1669150

Signature: t1:1669150:1417546223:9a41563560056b4ee139dcfb9ec993ba171bfe2f

Pulled By: @fredemmott