Fix docstring formatting nit in uuidsentinel

This doesn't actually appear in the docs because it's a private class,
but if we ever made it public we want it to format the code block
correctly.

Change-Id: Ia79b58d6d7847b65f946763c82a25ce8d8e581fb

oslo_utils/fixture.py

@@ -62,7 +62,7 @@ class _UUIDSentinels(object):
     referring to the same attribute will give the same UUID for the life of the
     instance. Plan accordingly.
 
-    Usage:
+    Usage::
 
         from oslo_utils.fixture import uuidsentinel as uuids
         ...