Flip strings for 'metamta.differential.unified-comment-context' config

Summary: These should be in (true, false) order, not (false, true) order. This was a transcritpion error in porting the config over.

Test Plan: Edited config from the web, verified the labels matched the underlying values.

Reviewers: tberman, btrahan

Reviewed By: tberman

CC: aran

Differential Revision: https://secure.phabricator.com/D5786