Copter: add auto calibration to ESC_CAL parameter description

No functional change

ArduCopter/Parameters.cpp

@@ -363,7 +363,7 @@ const AP_Param::Info Copter::var_info[] = {
     // @DisplayName: ESC Calibration
     // @Description: Controls whether ArduCopter will enter ESC calibration on the next restart.  Do not adjust this parameter manually.
     // @User: Advanced
-    // @Values: 0:Normal Start-up, 1:Start-up in ESC Calibration mode if throttle high, 2:Start-up in ESC Calibration mode regardless of throttle, 9:Disabled
+    // @Values: 0:Normal Start-up, 1:Start-up in ESC Calibration mode if throttle high, 2:Start-up in ESC Calibration mode regardless of throttle, 3:Start-up and automatically calibrate ESCs, 9:Disabled
     GSCALAR(esc_calibrate, "ESC_CALIBRATION",       0),
 
     // @Param: TUNE