Some header tag icons are too small

Summary: Ref D17991, this rule got more specific with shade tag re-write, so needs updating for headers.

Test Plan: Review a differential header, first icon is now 15px instead of 12px

Reviewers: epriestley, avivey

Reviewed By: avivey

Spies: Korvin

Differential Revision: https://secure.phabricator.com/D18472