docs: update ref attribute example (#2991)

* docs: update ref attribute example

ref -> useTemplateRef

* fix: add import of useTemplateRef.

#2991 (comment)

Author: Thy3634

src/api/built-in-special-attributes.md

@@ -58,9 +58,9 @@ Denotes a [template ref](/guide/essentials/template-refs).
 
   ```vue
   <script setup>
-  import { ref } from 'vue'
+  import { useTemplateRef } from 'vue'
 
-  const p = ref()
+  const pRef = useTemplateRef('p')
   </script>
 
   <template>