fix typo
fix typo

Summary: Closes https://github.com/facebook/folly/pull/692

Differential Revision: D6066294

Pulled By: yfeldblum

fbshipit-source-id: a533527058b8b5bc71fb6f40bad31a50e8c3f585

Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,rklabs/folly,facebook/folly,Orvid/folly,Orvid/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly