Fix broken option in jquery.wikibase.linkitem.js

This mistake was introduced in Ibae5bfc. It makes the client widget
unusable. Please backport this fix since it is on the live site as
far as I can tell.

Change-Id: Ia12eda93408a5d4a0f0efa2dcc45b3fbbdd51446
(cherry picked from commit b86b417aa6969d927056fc30409abd6cf748e8b5)