fix watchman website doc page favicon
fix watchman website doc page favicon

Summary: As title

Reviewed By: singhsrb

Differential Revision: D29926636

fbshipit-source-id: f06aa4c8ede9619c0c5ea8ffa85f42f62706e42a

facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman