Missing https:// in Google Group link

CONTRIBUTING.md

@@ -1,6 +1,6 @@
 ### Please read before contributing
 
-1) Do not post questions in the issues tracker. If you have any questions about Devise, search the [Wiki](https://github.com/plataformatec/devise/wiki) or use the [Mailing List](groups.google.com/group/plataformatec-devise) or [Stack Overflow](http://stackoverflow.com/questions/tagged/devise). 
+1) Do not post questions in the issues tracker. If you have any questions about Devise, search the [Wiki](https://github.com/plataformatec/devise/wiki) or use the [Mailing List](https://groups.google.com/group/plataformatec-devise) or [Stack Overflow](http://stackoverflow.com/questions/tagged/devise). 
 
 2) If you find a security bug, **DO NOT** submit an issue here. Please send an e-mail to [developers@plataformatec.com.br](mailto:developers@plataformatec.com.br) instead.