Fix a use of nonexistent .received_cancelled

.received_cancelled was changed to .cancelled() in
81edf5f; this change should have been
a part of that change.

src/python/grpcio_tests/tests/unit/_cython/_cancel_many_calls_test.py

@@ -43,7 +43,7 @@ def __init__(self):
 
 def _is_cancellation_event(event):
     return (event.tag is _RECEIVE_CLOSE_ON_SERVER_TAG and
-            event.batch_operations[0].received_cancelled)
+            event.batch_operations[0].cancelled())
 
 
 class _Handler(object):