Fix wrong availability for signal.SIGCHLD (#23285)

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.

Author: myzhang1029

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