Revert r42800 "* error.c: [DOC] Update bug tracker url by @ScotterC [Fixes GH-390]"

It should be fixed on www.ruby-lang.org side.
* [Bug #8854]
* ruby/www.ruby-lang.org#288 https://github.com/ruby/www.ruby-lang.org/issues/288

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42803 b2dd03c8-39d4-4d8f-98ff-823fe69b080e