update issue link to issues.dlang.org

Author: wilzbach

CONTRIBUTING.md

@@ -9,7 +9,7 @@ More Links
 ----------
 
 * Fork [on Github](https://github.com/dlang/phobos)
-* Use our [Bugzilla bug tracker](http://d.puremagic.com/issues/)
+* Use our [Bugzilla bug tracker](https://issues.dlang.org)
 * Follow the [D style](http://dlang.org/dstyle.html)
 * Participate in [our forum](http://forum.dlang.org/)
 * Ask questions on our `#d` IRC channel on freenode.org ([web interface](https://kiwiirc.com/client/irc.freenode.net/d))