Fix Wikidata extension name for Special:Version

the git commit hash linked is currently wrong, (it's
mediawiki/extensions.git not mediawiki/extensions/Wikidata.git)
perhaps because the extension name does not match
anything in gerrit.

This makes the name match the gerrit repo name.

Change-Id: Ic29b960de28040a4685e975abc737e56175ac92f