Trivial: Fix the comment of TempestConfigPrivate

During the review of I98e75ed52f754a45d0774067fd1713bee2cecb64
I was confused due to this original comment because that is a
class and it is impossible to use a class from the same class.
This patch fixes the comment.

Change-Id: Ib14e53f3824d03154c65cf0106b5c2e1f2df13d5
diff --git a/tempest/config.py b/tempest/config.py
index a2ccb84..ef8d5bd 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1072,7 +1072,7 @@
     return opt_list
 
 
-# this should never be called outside of this class
+# This should never be called outside of this module
 class TempestConfigPrivate(object):
     """Provides OpenStack configuration information."""