CUSTOM_USER_BUTTONS followup (#21284)

Followup to #18389

Marlin/Configuration_adv.h

 //#define CUSTOM_USER_BUTTONS
 #if ENABLED(CUSTOM_USER_BUTTONS)
   //#define BUTTON1_PIN -1
-  #if PIN_EXISTS(BUTTON1_PIN)
+  #if PIN_EXISTS(BUTTON1)
     #define BUTTON1_HIT_STATE     LOW       // State of the triggered button. NC=LOW. NO=HIGH.
     #define BUTTON1_WHEN_PRINTING false     // Button allowed to trigger during printing?
     #define BUTTON1_GCODE         "G28"
   #endif
 
   //#define BUTTON2_PIN -1
-  #if PIN_EXISTS(BUTTON2_PIN)
+  #if PIN_EXISTS(BUTTON2)
     #define BUTTON2_HIT_STATE     LOW
     #define BUTTON2_WHEN_PRINTING false
     #define BUTTON2_GCODE         "M140 S" STRINGIFY(PREHEAT_1_TEMP_BED) "\nM104 S" STRINGIFY(PREHEAT_1_TEMP_HOTEND)
   #endif
 
   //#define BUTTON3_PIN -1
-  #if PIN_EXISTS(BUTTON3_PIN)
+  #if PIN_EXISTS(BUTTON3)
     #define BUTTON3_HIT_STATE     LOW
     #define BUTTON3_WHEN_PRINTING false
     #define BUTTON3_GCODE         "M140 S" STRINGIFY(PREHEAT_2_TEMP_BED) "\nM104 S" STRINGIFY(PREHEAT_2_TEMP_HOTEND)