doc: posix: signal: mark sigprocmask as supported

`sigprocmask()` is now implemented, mark it so.

Signed-off-by: Yong Cong Sin <[email protected]>

Author: ycsin

doc/services/portability/posix/option_groups/index.rst

@@ -197,7 +197,7 @@ required for error and event handling.
     sigismember(),yes
     signal(),
     sigpending(),
-    sigprocmask(),
+    sigprocmask(),yes
     igsuspend(),
     sigwait(),
     strsignal(),yes