Fix erroneous attribute name.

This defect was introduced in 9e4d0610ea5e2bb302c. I had thought that
this code was exercised in tests but it is bypassed by the use of
grpc_test.beta.test_utilities.not_really_secure_channel. :-(

src/python/grpcio/grpc/beta/implementations.py

@@ -147,7 +147,7 @@ def secure_channel(host, port, client_credentials):
     A secure Channel to the remote host through which RPCs may be conducted.
   """
   intermediary_low_channel = _intermediary_low.Channel(
-      '%s:%d' % (host, port), client_credentials.intermediary_low_credentials)
+      '%s:%d' % (host, port), client_credentials._intermediary_low_credentials)
   return Channel(intermediary_low_channel._internal, intermediary_low_channel)  # pylint: disable=protected-access