Fix SyntaxHighlight MediaWiki extension name

Summary:
Extension has been renamed SyntaxHighlight but repository and extension name
are still SyntaxHighlight_GeSHi.

This is a follow-up to change ffef49bf.

Test Plan: Tested on agora.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1453