[Docs] Update Super ALT↯TAB to remove use of unregister_code16 (#5766)

The code as originally listed didn't work for me, but replacing `unregister_code16(LALT(KC_TAB));` with `unregister_code(KC_LALT);` fixes the problem and causes the macro to work as intended.

Thanks to folks on Discord for helping me figure this out.

docs/feature_macros.md

@@ -230,7 +230,7 @@ bool process_record_user(uint16_t keycode, keyrecord_t *record) {
 void matrix_scan_user(void) {     # The very important timer. 
   if (is_alt_tab_active) {
     if (timer_elapsed(alt_tab_timer) > 1000) {
-      unregister_code16(LALT(KC_TAB));
+      unregister_code(KC_LALT);
       is_alt_tab_active = false;
     }
   }