Update COPYING

Convert to wikitext because of bug https://bugzilla.wikimedia.org/show_bug.cgi?id=66767

Follow up to commit https://github.com/SemanticMediaWiki/SemanticWatchlist/commit/b7e750de199d7323305657e1253abfefa8845c26
Merge branch 'master' of ssh://wmfgerrit/mediawiki/extensions/SemanticWatchlist