Add additional CSS class when code-block is counterexample

Summary: Ref T7444, there is no means to style the header without another CSS rule in place.

Test Plan: regenerate a Phriction page, see new class

Reviewers: epriestley, btrahan

Subscribers: Korvin, epriestley

Maniphest Tasks: T7444

Differential Revision: https://secure.phabricator.com/D12594