Format sql in db.sqlalchemy.session docstring

Some SQL in the module wasn't marked as such so it wasn't
nicely highlighted.

Change-Id: If902796886cb614262b305b221fc9890d3fd1c87

openstack/common/db/sqlalchemy/session.py

@@ -124,7 +124,9 @@ Recommended ways to use sessions within this framework:
                 filter_by(id=subq.as_scalar()).
                 update({'bar': newbar}))
 
-  For reference, this emits approximately the following SQL statement::
+  For reference, this emits approximately the following SQL statement:
+
+  .. code:: sql
 
     UPDATE bar SET bar = ${newbar}
         WHERE id=(SELECT bar_id FROM foo WHERE id = ${foo_id} LIMIT 1);