bpo-44353: Correct docstring for NewType (GH-29785)

The docstring for `typing.NewType` is no longer strictly accurate, now that `NewType` is a class, rather than a function
(cherry picked from commit 93c65df)

Co-authored-by: Alex Waygood <[email protected]>

Author: miss-islington

Lib/typing.py

@@ -2427,7 +2427,7 @@ class NewType:
     """NewType creates simple unique types with almost zero
     runtime overhead. NewType(name, tp) is considered a subtype of tp
     by static type checkers. At runtime, NewType(name, tp) returns
-    a dummy function that simply returns its argument. Usage::
+    a dummy callable that simply returns its argument. Usage::
 
         UserId = NewType('UserId', int)