Fix typo in flatten.rs
Fix typo in flatten.rs

Reviewed By: tyao1

Differential Revision: D25560261

fbshipit-source-id: cca633b4b5d1d59c22c6f625232b89872c6cf2ab

voideanvalue/relay,wincent/relay,kassens/relay,voideanvalue/relay,xuorig/relay,kassens/relay,facebook/relay,josephsavona/relay,yungsters/relay,wincent/relay,kassens/relay,facebook/relay,atxwebs/relay,wincent/relay,atxwebs/relay,wincent/relay,facebook/relay,voideanvalue/relay,voideanvalue/relay,josephsavona/relay,wincent/relay,facebook/relay,xuorig/relay,facebook/relay,wincent/relay,kassens/relay,yungsters/relay,xuorig/relay,xuorig/relay,facebook/relay,xuorig/relay,kassens/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,yungsters/relay,atxwebs/relay,atxwebs/relay,kassens/relay,josephsavona/relay