Fix the 'service' decorator path in doc

Documentation update.

Change-Id: I3bca9de9a2e3fbc12647a1c55be0efc4434df2c0
diff --git a/HACKING.rst b/HACKING.rst
index 79ebc4d..8407734 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -103,10 +103,10 @@
 Service Tagging
 ---------------
 Service tagging is used to specify which services are exercised by a particular
-test method. You specify the services with the ``tempest.test.services``
+test method. You specify the services with the ``tempest.common.utils.services``
 decorator. For example:
 
-@services('compute', 'image')
+@utils.services('compute', 'image')
 
 Valid service tag names are the same as the list of directories in tempest.api
 that have tests.