Update COPYING

Convert to wikitext because of [bug 66767](https://bugzilla.wikimedia.org/show_bug.cgi?id=66767)

Follow up to commit https://github.com/SemanticMediaWiki/SemanticExtraSpecialProperties/commit/6a4129cc24b958b4872bda6822b4eb089415983e
Merge branch 'master' of ssh://wmfgerrit/mediawiki/extensions/SemanticExtraSpecialProperties