Fix typo in gradle.md
Fix typo in gradle.md

Summary: Closes https://github.com/facebook/litho/pull/251

Differential Revision: D6121475

Pulled By: passy

fbshipit-source-id: c37c26acd4c4ce05ee06f800a0bb3a2c144d1d6d

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