📝 Update ExtUI example

Marlin/src/lcd/extui/example/example.cpp

   #if HAS_PID_HEATING
     void onPidTuning(const result_t rst) {
       // Called for temperature PID tuning result
+      switch (rst) {
+        case PID_BAD_EXTRUDER_NUM: break;
+        case PID_TEMP_TOO_HIGH:    break;
+        case PID_TUNING_TIMEOUT:   break;
+        case PID_DONE:             break;
+      }
     }
   #endif