Update template-refs.md (vuejs#1450)

Remove unreasonable `<script setup>` in `options-api`

Author: Rainer-Yu

src/guide/essentials/template-refs.md

@@ -38,7 +38,7 @@ onMounted(() => {
 The resulting ref is exposed on `this.$refs`:
 
 ```vue
-<script setup>
+<script>
 export default {
   mounted() {
     this.$refs.input.focus()