docs: Update key-attribute.md (#1045)

* Update key-attribute.md

* Update key-attribute.md

Author: Alex-Sokolov

src/guide/migration/key-attribute.md

@@ -59,8 +59,8 @@ In Vue 2.x, a `<template>` tag could not have a `key`. Instead, you could place
 ```html
 <!-- Vue 2.x -->
 <template v-for="item in list">
-  <div :key="item.id">...</div>
-  <span :key="item.id">...</span>
+  <div :key="'heading-' + item.id">...</div>
+  <span :key="'content-' + item.id">...</span>
 </template>
 ```