Update style guide link in HACKING.rst

The previous github HACKING.rst doesn't exist anymore, so this commit
just updates the link in tempest's HACKING.rst to point it to the
correct location.

Change-Id: If49f938528ac9059a8e1983b281081fc6137b700
diff --git a/HACKING.rst b/HACKING.rst
index 9878b67..499b436 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -2,7 +2,7 @@
 ====================
 
 - 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
 
 Tempest Specific Commandments