Update developing.rst

Use unittest.mock instead of mock

Change-Id: Ib573e9d217b4f18ef4e7ba3ab581164be423cb26

Author: likui

doc/source/contributor/developing.rst

@@ -199,7 +199,6 @@ Example
 
     import copy
     import fixtures
-    import mock
     import os
 
     from osc_lib.api import auth
@@ -208,4 +207,5 @@ Example
 
     from openstackclient import shell
     from openstackclient.tests import utils
+    from unittest import mock