sd-daemon: don't mention strerror_safe() in examples in public headers

It's an internal function we define, noone else should bother.

src/systemd/sd-daemon.h

@@ -260,7 +260,7 @@ int sd_notify(int unset_environment, const char *state);
 
      sd_notifyf(0, "STATUS=Failed to start up: %s\n"
                    "ERRNO=%i",
-                   strerror_safe(errno),
+                   strerror(errno),
                    errno);
 
   See sd_notifyf(3) for more information.