[3.13] Docs: improve generic typing.NamedTuple example (GH-124739) (#124764)

Docs: improve generic `typing.NamedTuple` example (GH-124739)
(cherry picked from commit 76fbee6)

Co-authored-by: CBerJun <[email protected]>

Author: miss-islington

Doc/library/typing.rst

@@ -2273,7 +2273,9 @@ types.
 
    Backward-compatible usage::
 
-       # For creating a generic NamedTuple on Python 3.11 or lower
+       # For creating a generic NamedTuple on Python 3.11
+       T = TypeVar("T")
+
        class Group(NamedTuple, Generic[T]):
            key: T
            group: list[T]