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

src/applications/phriction/markup/PhrictionRemarkupRule.php

@@ -275,7 +275,7 @@ final class PhrictionRemarkupRule extends PhutilRemarkupRule {
     }
 
     if ($context instanceof PhrictionDocument) {
-      return $context->getSlug();
+      return $context->getContent()->getSlug();
     }
 
     return null;