fixed incorrect key combination for locking screen on macOS

keyboards/planck/keymaps/callum/keymap.c

@@ -137,11 +137,11 @@ bool process_record_user(uint16_t keycode, keyrecord_t *record) {
     case LOCK:
       if (record->event.pressed) {
         register_code(KC_RSFT);
-        register_code(KC_RGUI);
+        register_code(KC_RCTL);
         register_code(KC_POWER);
       } else {
         unregister_code(KC_POWER);
-        unregister_code(KC_RGUI);
+        unregister_code(KC_RCTL);
         unregister_code(KC_RSFT);
       }
       return false;