Fix external Projects Wikis Remarkup links

Summary: Broken by D2110 and D2111.

Test Plan:
Disable differential.custom-remarkup-rules which tricked my original test plan.

[[ http://example.com | test ]]

Reviewers: Koolvin, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T1090

Differential Revision: https://secure.phabricator.com/D2164