Make mediawiki.content.json.css not trigger JSDuck

Fun fact: JSDuck tests don't run for .css file changes on Jenkins, but
when they do run for .js file changes, they also process .css files
and the documentation comments in them.

Change /** to /*! to prevent that, like we've done in other files.

Follow-up to d2a82fcb.

Change-Id: Ib7f9875ea818bc72fe9aaed167b428e570c6df2e