Fix incorrect assertion message.

colour/utilities/network.py

@@ -1452,7 +1452,7 @@ def set_output(self, name: str, value: Any) -> None:
 
         attest(
             name in self._output_ports,
-            f'"{name}" is not a member of "{self._name}" input ports!',
+            f'"{name}" is not a member of "{self._name}" output ports!',
         )
 
         self._output_ports[name].value = value