Add metaclass explicitly

Author: hoodmane

peps/pep-0818.rst

@@ -945,7 +945,7 @@ The ``JSProxy`` base class has the following shape:
 
 .. code:: python
 
-    class JSProxy:
+    class JSProxy(metaclass=_JsProxyMetaClass):
         def __eq__(self, other: JSProxy) -> bool:
             ...