Fix grammar in layout docs
Fix grammar in layout docs

Summary: Fixes facebook/litho#122.

Reviewed By: emilsjolander

Differential Revision: D5431163

fbshipit-source-id: 8aa86a6da8454ac7a7b1600e8e2f0975a3bf5cf4

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho