man page: Correct option letter for DBus

Signed-off-by: Ferenc Wágner <wferi@niif.hu>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

man/corosync-notifyd.8

@@ -78,7 +78,7 @@ corosync-notifyd[18505]: r2[1550100672] ip:192.168.100.92 left
 .br
 
 .br
-$ corosync-notifyd -o
+$ corosync-notifyd -d
 .br
 
 Note this output below is from "dbus-monitor --system"