Fix headings for "Desktop Plugin API"
Fix headings for "Desktop Plugin API"

Reviewed By: nikoant

Differential Revision: D36508504

fbshipit-source-id: ece204e5a18d2c98481e21f5c6f0dc42386cf2b5

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper