Updates OpenStack Style Commandments link

The current link in the HACKING file is broken. This references the
correct location for contributors to view.

Change-Id: I3cd4841035400d09a3e3e3369bb3a2a8c4bdee30

HACKING.rst

@@ -2,7 +2,7 @@ Nova Style Commandments
 =======================
 
 - Step 1: Read the OpenStack Style Commandments
-  https://github.com/openstack-dev/hacking/blob/master/HACKING.rst
+  http://docs.openstack.org/developer/hacking/
 - Step 2: Read on
 
 Nova Specific Commandments