Update 'test_volume_upload' test docstring

'image_client' was renamed to 'images_client'

Change-Id: Ia190702c1b8144e936cd528827193b3516f72623
diff --git a/tempest/api/volume/test_volumes_actions.py b/tempest/api/volume/test_volumes_actions.py
index 2ed2a06..0e7f1e9 100644
--- a/tempest/api/volume/test_volumes_actions.py
+++ b/tempest/api/volume/test_volumes_actions.py
@@ -99,7 +99,7 @@
         # NOTE(gfidente): the volume uploaded in Glance comes from setUpClass,
         # it is shared with the other tests. After it is uploaded in Glance,
         # there is no way to delete it from Cinder, so we delete it from Glance
-        # using the Glance image_client and from Cinder via tearDownClass.
+        # using the Glance images_client and from Cinder via tearDownClass.
         image_name = data_utils.rand_name(self.__class__.__name__ + '-Image')
         body = self.volumes_client.upload_volume(
             self.volume['id'], image_name=image_name,