gh-150232: update Thread group parameter doc (GH-150283)

(cherry picked from commit 82191c6)

Co-authored-by: My-ABC <569817555@qq.com>

Author: My-ABC

Doc/library/threading.rst

@@ -515,7 +515,7 @@ since it is impossible to detect the termination of alien threads.
    This constructor should always be called with keyword arguments.  Arguments
    are:
 
-   *group* should be ``None``; reserved for future extension when a
+   *group* must be ``None`` as it is reserved for future extension when a
    :class:`!ThreadGroup` class is implemented.
 
    *target* is the callable object to be invoked by the :meth:`run` method.