update link to discuss mailing list (#507)

Author: tobyhodges

bin/boilerplate/CONTRIBUTING.md

@@ -137,7 +137,7 @@ You can also [reach us by email][email].
 [dc-issues]: https://github.com/issues?q=user%3Adatacarpentry
 [dc-lessons]: http://datacarpentry.org/lessons/
 [dc-site]: http://datacarpentry.org/
-[discuss-list]: http://lists.software-carpentry.org/listinfo/discuss
+[discuss-list]: https://carpentries.topicbox.com/groups/discuss
 [github]: https://github.com
 [github-flow]: https://guides.github.com/introduction/flow/
 [github-join]: https://github.com/join