Fix subunit-describe-calls name of usage

This commit fixes to show 'subunit-describe-calls' instead of 'Argument
Parser' in its usage.

Change-Id: I62e64022285aad4e5bf5943451e93d8f674b238c
diff --git a/tempest/cmd/subunit_describe_calls.py b/tempest/cmd/subunit_describe_calls.py
index da7f426..9391823 100644
--- a/tempest/cmd/subunit_describe_calls.py
+++ b/tempest/cmd/subunit_describe_calls.py
@@ -203,7 +203,7 @@
         desc = "Outputs all HTTP calls a given test made that were logged."
         super(ArgumentParser, self).__init__(description=desc)
 
-        self.prog = "Argument Parser"
+        self.prog = "subunit-describe-calls"
 
         self.add_argument(
             "-s", "--subunit", metavar="<subunit file>", required=True,