doc: Update the module of services decorator

The services decorator has been moved to the utils module by [1].

[1] cd36841ca25b39b9c8ad1b83e0abd0a191d538a0

Change-Id: I9a94c3e46a1e72d96a104996c91eb9233f8c79ee
diff --git a/doc/source/index.rst b/doc/source/index.rst
index ed1a773..3e6528a 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -260,8 +260,8 @@
        def test_something_else(self):
            pass
 
-@test.services
-~~~~~~~~~~~~~~
+@utils.services
+~~~~~~~~~~~~~~~
 
 The `services` decorator is used to indicate which services are exercised by
 a given test. The `services` decorator may only be used on scenario tests, and
@@ -274,7 +274,7 @@
 .. code-block:: python
 
    class ReverseTest(BaseDnsTest):
-       @test.services('network')
+       @utils.services('network')
        def test_reverse_dns_for_fips(self):
            pass