Doc: add the missing ".tp_flags" in type definition (GH-12902)

Doc/extending/newtypes_tutorial.rst

@@ -92,6 +92,7 @@ The second bit is the definition of the type object. ::
        .tp_doc = "Custom objects",
        .tp_basicsize = sizeof(CustomObject),
        .tp_itemsize = 0,
+       .tp_flags = Py_TPFLAGS_DEFAULT,
        .tp_new = PyType_GenericNew,
    };