Fix broken heading of Vue 3 migration guide doc

changelogs/unreleased/fix-broken-heading-of-vue-3-migration-guide-doc.yml

+---
+title: Fix broken heading of Vue 3 migration guide doc
+merge_request: 31951
+author: Gilang Gumilar
+type: fixed

doc/development/fe_guide/vue3_migration.md

@@ -58,7 +58,7 @@ export default createEventHub();
 Event hubs created with the factory expose the same methods as Vue 2 event hubs (`$on`, `$once`, `$off` and
 `$emit`), making them backward compatible with our previous approach.
 
-## <template functional>
+## \<template functional>
 
 **Why?**