Doc: Fix broken manpage link (GH-21937)

sigprocmask is in section 2, not 3.
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 05b285e..00a730b 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -416,7 +416,7 @@
 
    :data:`SIGKILL` and :data:`SIGSTOP` cannot be blocked.
 
-   .. availability:: Unix.  See the man page :manpage:`sigprocmask(3)` and
+   .. availability:: Unix.  See the man page :manpage:`sigprocmask(2)` and
       :manpage:`pthread_sigmask(3)` for further information.
 
    See also :func:`pause`, :func:`sigpending` and :func:`sigwait`.