Fix unit test failure in link-alternate

Summary: arc unit --everything triggers a FAIL PhutilRemarkupEngineTestCase::testEngine due to adding the CSS class remarkup-link-ext in rP349f006904fabf1d4df31ff4840502af3ab379a7. Thus update it accordingly.

Test Plan: Run arc unit.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25820