📝 Fix DEFAULT_DUAL_X_CARRIAGE_MODE comment

Marlin/Configuration_adv.h

                       // without modifying the firmware (through the "M218 T1 X???" command).
                       // Remember: you should set the second extruder x-offset to 0 in your slicer.
 
-  // This is the default power-up mode which can be later using M605.
+  // This is the default power-up mode which can be changed later using M605 S<mode>.
   #define DEFAULT_DUAL_X_CARRIAGE_MODE DXC_AUTO_PARK_MODE
 
   // Default x offset in duplication mode (typically set to half print bed width)