fix Vary Subjects option names

Summary:
The option names for Vary Subjects are copypasta from the Add "Re:" Prefix option. Fix their names to refer to Vary Subjects instead.

Fixes T11148

Test Plan: Verify option names for Vary Subjects refer to Add "Re:" Prefix before apply. Verify they no longer do after apply.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T11148

Differential Revision: https://secure.phabricator.com/D16113