Fix px4io bind failed message to say safet is off instead of armed. (#5834)

Minor message clarity fix.

src/drivers/px4io/px4io.cpp

@@ -1700,7 +1700,7 @@ PX4IO::dsm_bind_ioctl(int dsmMode)
 		}
 
 	} else {
-		mavlink_log_info(&_mavlink_log_pub, "[IO] system armed, bind request rejected");
+		mavlink_log_info(&_mavlink_log_pub, "[IO] safety off, bind request rejected");
 	}
 }