t1006: adjust test description

Problem: One of the descriptions of one of the unit tests in
t1006_job_archive.py is incorrectly stating that one of the function
calls should raise an error, but this is not the case; it should just
return a list of length 0.

Adjust the test description to more accurately explain what the expected
behavior is.

t/python/t1006_job_archive.py

@@ -161,7 +161,7 @@ def test_03_after_start_time_all(self):
         self.assertEqual(len(job_records), 18)
 
     # passing a timestamp after all of the start time
-    # of all the completed jobs should return a failure message
+    # of all the completed jobs should return no jobs
     @mock.patch("time.time", mock.MagicMock(return_value=11000000))
     def test_04_after_start_time_none(self):
         my_dict = {"after_start_time": time.time()}