Fix underline length to fix docs build

Author: hugovk

Doc/howto/regex.rst

@@ -1279,7 +1279,7 @@ can be solved with a faster and simpler string method.
 .. _match-versus-search:
 
 prefixmatch() (aka match) versus search()
------------------------------
+-----------------------------------------
 
 :func:`~re.prefixmatch` was added in Python 3.15 as the :ref:`preferred name
 <prefixmatch-vs-match>` for :func:`~re.match`. Before this, it was only known