Fix `LINT_KEYWORD_CASING` for `class` keyword

Summary: D10653 didn't quite cut it.

Test Plan: Tested on a real file.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10656