Fix last child selector in phabricator-remarkup divs

Summary: Fixes T9793, the selector is triggering on the last child of every child of phabricator-remarkup, not the last child of phabricator-remarkup

Test Plan: Checkout latest changelog

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9793

Differential Revision: https://secure.phabricator.com/D14544