R1:66a7ca49b20c

Fix incorrect context extraction for relative Phriction links on Phriction pages

Summary: Ref T13077. This content extraction rule wasn't right and caused rendering on Phriction pages to extract context improperly.

Test Plan: Viewed pages in Phriction with relative links to other documents.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19114