Update Doc/whatsnew/3.14.rst

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: corona10

Doc/whatsnew/3.14.rst

@@ -798,7 +798,7 @@ in CPython by Mark Shannon, Garrett Gu, Haoran Xu, and Josh Haberman.)
 .. _whatsnew314-free-threaded-cpython:
 
 Free-threaded mode
----------------------
+------------------
 
 Free-threaded mode (:pep:`703`), initially added in 3.13, has been significantly improved. The implementation described in PEP 703 was finished, including C API changes, and temporary workarounds in the interpreter were replaced with more permanent solutions. The specializing adaptive interpreter (:pep:`659`) is now enabled in free-threaded mode, which along with many other optimizations greatly improves its performance. The performance penalty on single-threaded code in free-threaded mode is now roughly 5-10%, depending on platform and C compiler used.