[3.12] gh-111729: update generic syntax for typing.Concatenate sample code in Doc/library/typing.rst (GH-111734) (#111814)

(cherry picked from commit c3e19c3a62)

Co-authored-by: 方糖 <cubesugarcheese@qq.com>

Doc/library/typing.rst

@@ -1135,16 +1135,13 @@ These can be used as types in annotations. They all support subscription using
 
       from collections.abc import Callable
       from threading import Lock
-      from typing import Concatenate, ParamSpec, TypeVar
-
-      P = ParamSpec('P')
-      R = TypeVar('R')
+      from typing import Concatenate
 
       # Use this lock to ensure that only one thread is executing a function
       # at any time.
       my_lock = Lock()
 
-      def with_lock(f: Callable[Concatenate[Lock, P], R]) -> Callable[P, R]:
+      def with_lock[**P, R](f: Callable[Concatenate[Lock, P], R]) -> Callable[P, R]:
           '''A type-safe decorator which provides a lock.'''
           def inner(*args: P.args, **kwargs: P.kwargs) -> R:
               # Provide the lock as the first argument.