What if 8 was 9? (#20076)

Marlin/src/MarlinCore.cpp

     kill();
   }
 
-  // M18 / M94 : Handle steppers inactive time timeout
+  // M18 / M84 : Handle steppers inactive time timeout
   if (gcode.stepper_inactive_time) {
 
     static bool already_shutdown_steppers; // = false