Adjust migration section grouping title to better reflect its contents (vuejs#286)

**Motivation**

* Pages under this section group reflect changes that happened only between Vue 2 and Vue 3. It does not help if you're migrating from Vue 1 for example.
* Since it is already a Vue 3 documentation calling migration section `Migration to Vue 3` is a bit of a tautology.
* This grouping does not reflect migration from other frameworks like React or Svelte.

Author: CyberAP

src/.vuepress/config.js

@@ -108,7 +108,7 @@ const sidebar = {
       ]
     },
     {
-      title: 'Migration to Vue 3',
+      title: 'Migration from Vue 2',
       collapsable: true,
       children: [
         'migration/introduction',