bpo-38391: Fixing a typo for Py_DECREF (GH-16616)

Doc/extending/newtypes_tutorial.rst

@@ -182,7 +182,7 @@ set to *NULL*. ::
    Py_INCREF(&CustomType);
    if (PyModule_AddObject(m, "Custom", (PyObject *) &CustomType) < 0) {
        Py_DECREF(&CustomType);
-       PY_DECREF(m);
+       Py_DECREF(m);
        return NULL;
    }