Remove optional namespace prefixes from <gallery> example

The namespace is optional since at least 2011, see
https://www.mediawiki.org/wiki/Special:Code/MediaWiki/79639

More and more wikis switch to the syntax without repeating the
namespace over and over again. Main advantage: This avoids all
discussions which alias (localized or what) should be used.
Example discussion:
https://de.wikipedia.org/wiki/WP:VV#Erweiterte_Bearbeiten-Werkzeugleiste_-_Bildgalerie_einf.C3.BCgen

This change keeps the parameter in the JavaScript so it still can
be used in local translations and existing local translations don't
trick users into inserting $1's into the edit window.

Change-Id: I31826d1a3db51f22f46ae469864a23d7c8298e09
Merge "Remove optional namespace prefixes from <gallery> example"