Fix wrong availability for signal.SIGCHLD (GH-23285) (#23426)

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.
(cherry picked from commit 4c24b08)

Co-authored-by: Zhang Maiyun <[email protected]>

Co-authored-by: Zhang Maiyun <[email protected]>

Author: miss-islington

Doc/library/signal.rst

@@ -117,7 +117,7 @@ The variables defined in the :mod:`signal` module are:
 
    Child process stopped or terminated.
 
-   .. availability:: Windows.
+   .. availability:: Unix.
 
 .. data:: SIGCLD