Fix wrong availability for signal.SIGCHLD (GH-23285) (#23425)

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.
(cherry picked from commit 4c24b08cd3239b417a5f8c7ba2ba54c840d051e3)

Co-authored-by: Zhang Maiyun <myzhang1029@hotmail.com>

Co-authored-by: Zhang Maiyun <myzhang1029@hotmail.com>
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 05b285e..e1daeff 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