Fix Phriction link syntax a little more

Summary: This still wasn't quite right -- a link like [[ Porcupine Facts ]] with a space would not lookup correctly, and would render as porcupine_facts.

Test Plan: Verified that [[ Porcupine Facts ]] now works correctly.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15780