message: docs: improve useCustomFieldValue demo
diff: @@ -14,11 +14,16 @@ If you want to custom Form items, you can insert your component into the `input`
  ```js
  // MyComponent.vue
  +import { ref } from 'vue';
  import { useCustomFieldValue } from '@vant/use';
  export default {
  setup() {
  - useCustomFieldValue(() => 'Some value');
  + const myValue = ref(0);
  +
  + useCustomFieldValue(() => myValue.value);
  +
  + return { myValue };
  },
  };
  ```
  