doc: posix: signal: fix sigsuspend typo

The `sigsuspend` is currently `igsuspend`, fix that.

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

Author: ycsin

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

@@ -198,7 +198,7 @@ required for error and event handling.
     signal(),
     sigpending(),
     sigprocmask(),yes
-    igsuspend(),
+    sigsuspend(),
     sigwait(),
     strsignal(),yes