PEP 526: Fix name mangling example (#5138)

Name mangling only applies inside a class body, so a module-level
``__points: int`` is stored as ``'__points'``, not ``'_Player__points'``.
Move the annotation into the class and print its ``__annotations__``.

Author: jang-hs

peps/pep-0526.rst

@@ -438,13 +438,13 @@ Here is an example::
 
   from typing import Dict
   class Player:
-      ...
+      __points: int
   players: Dict[str, Player]
-  __points: int
 
   print(__annotations__)
-  # prints: {'players': typing.Dict[str, __main__.Player],
-  #          '_Player__points': <class 'int'>}
+  # prints: {'players': typing.Dict[str, __main__.Player]}
+  print(Player.__annotations__)
+  # prints: {'_Player__points': <class 'int'>}
 
 ``__annotations__`` is writable, so this is permitted::