docs: add onMounted lifecycle hook to custom field rendering tutorial

Author: NoOne7135

adminforth/documentation/docs/tutorial/03-Customization/02-customFieldRendering.md

@@ -512,7 +512,7 @@ Sometimes standard filters are not enough, and you want to make a convenient UI
 </template>
 
 <script setup lang="ts">
-import { ref, watch } from 'vue';
+import { ref, watch, onMounted } from 'vue';
 
 const emit = defineEmits(['update:modelValue']);