Update COPYING

follow up to commit https://github.com/JeroenDeDauw/Validator/commit/798583377b482fc3082958d08937feadbb9a3143#diff-697541437a6bbe5a1fc9ac1caededd67. This file's content must be in wikitext.
Merge pull request #4 from kghbln/patch-1

Update COPYING
Merge branch 'master' of ssh://wmfgerrit/mediawiki/extensions/Validator