Fix typos in README.md
Fix typos in README.md

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1141

Reviewed By: LeeHowes

Differential Revision: D15560029

Pulled By: yfeldblum

fbshipit-source-id: 5f0d5ee8d09be834d1216d4c39244e0855d2dc0f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly