cpuidle: fix comment

Signed-off-by: Antonio Ospite <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: "[e-mail]" <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/cpuidle/governors/menu.c

···
 	 *
 	 * Any measured amount of time will include the exit latency.
 	 * Since we are interested in when the wakeup begun, not when it
-	 * was completed, we must substract the exit latency. However, if
+	 * was completed, we must subtract the exit latency. However, if
 	 * the measured amount of time is less than the exit latency,
 	 * assume the state was never reached and the exit latency is 0.
 	 */