Add missing i18n message.

I created a pull-request to upstream the changes, and I opened an issue
asking robla to migrate the extension to Gerrit. Until then, I'll make
the change here.

https://github.com/robla/mediawiki-jsondata/issues/1
https://github.com/robla/mediawiki-jsondata/issues/2

Change-Id: I8a68f1ff5be028295ad438bfdc65d1ffb918704e