message: Fix datagram_servant_passivated_msg documentation
  The documentation incorrectly refers to datagram_servant_closed_msg
  instead of datagram_servant_passivated_msg.
diff: @@ -253,7 +253,7 @@ handles is no longer valid.
  datagram_handle handle;
  };
  -A ``datagram_servant_closed_msg`` informs the broker that one of its
  +A ``datagram_servant_passivated_msg`` informs the broker that one of its
  handles entered passive mode and no longer accepts new data trigger_.
  .. _trigger:
  