fix (#162): incorrect comment type

Author: bencodezen

src/guide/migration/fragments.md

@@ -9,7 +9,7 @@ In Vue 3, components now have official support for multi-root node components, i
 In 2.x, multi-root components were not supported and would emit a warning when a user accidentally created one. As a result, many components are wrapped in a single `<div>` in order to fix this error.
 
 ```html
-// Layout.vue
+<!-- Layout.vue -->
 <template>
   <div>
     <header>...</header>