ntp: fix undefined variable in error print

The variable "$SCRIPTNAME" is undefined; replace with "$0".

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ntp/S49ntp

@@ -36,7 +36,7 @@ case "$1" in
 	sleep 1
 	$0 start
 	;;
-  *) echo "Usage: $SCRIPTNAME {start|stop|restart|reload|force-reload}" >&2
+  *) echo "Usage: $0 {start|stop|restart|reload|force-reload}" >&2
 	exit 1
 	;;
 esac