Fix wrong availability for signal.SIGCHLD (#23285)

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 00a730b..84a569d 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -117,7 +117,7 @@
 
    Child process stopped or terminated.
 
-   .. availability:: Windows.
+   .. availability:: Unix.
 
 .. data:: SIGCLD