Update free-threading-extensions.rst

The preferred option for building free-threaded wheels has changed from `CIBW_FREE_THREADED_SUPPORT` to `CIBW_ENABLE` with `cpython-freethreading`.

The anchor to the existing link no longer work is and updated to the new anchor.

Author: 3willows

Doc/howto/free-threading-extensions.rst

@@ -395,8 +395,8 @@ The wheels, shared libraries, and binaries are indicated by a ``t`` suffix.
 * `pypa/manylinux <https://github.com/pypa/manylinux>`_ supports the
   free-threaded build, with the ``t`` suffix, such as ``python3.13t``.
 * `pypa/cibuildwheel <https://github.com/pypa/cibuildwheel>`_ supports the
-  free-threaded build if you set
-  `CIBW_FREE_THREADED_SUPPORT <https://cibuildwheel.pypa.io/en/stable/options/#free-threaded-support>`_.
+  free-threaded build if you set 
+  `CIBW_ENABLE with cpython-freethreading <https://cibuildwheel.pypa.io/en/stable/options/#enable>`_.
 
 Limited C API and Stable ABI
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