Fix title underline length in whatisnew doc.

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Author: stroxler

Doc/whatsnew/3.12.rst

@@ -351,7 +351,7 @@ The :class:`tempfile.NamedTemporaryFile` function has a new optional parameter
 *delete_on_close* (Contributed by Evgeny Zorin in :gh:`58451`.)
 
 typing
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+------
 
 * Add :func:`typing.override`, an override decorator telling to static type
   checkers to verify that a method overrides some method or attribute of the