Copter: Change Tradheli arming failure message.

"stop rotor spinning" is not a good indication of what the problem really is.  The user needs to understand they must switch off the rotor speed control switch.  I would prefer to say "Rotor Speed Control Engaged" but we are limited in message length.

ArduCopter/motors.pde

@@ -666,7 +666,7 @@ static bool arm_checks(bool display_failure, bool arming_from_gcs)
     // heli specific arming check
     if (!motors.allow_arming()){
         if (display_failure) {
-            gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: stop Rotor spinning"));
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("Arm: Rotor Control Engaged"));
         }
         return false;
     }