Fix comment explaining how to use thenValue/thenError.
Fix comment explaining how to use thenValue/thenError.

Summary: I thought that since I noticed it, and it tripped me up a bit, we might as well fix it.

Reviewed By: LeeHowes

Differential Revision: D26048767

fbshipit-source-id: 91ba9f834df411eb2d775691be4aec8a8d99776b

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly