Correct a comment in test_round_trip_time.

Author: ajdavis

test/test_topology.py

@@ -264,7 +264,7 @@ def raises_err():
         wait_until(raises_err, 'discover server is down')
         self.assertIsNone(s.description.round_trip_time)
 
-        # Bring it back, RTT is now 30 milliseconds.
+        # Bring it back, RTT is now 20 milliseconds.
         available = True
         round_trip_time = 20