Update index.md (#2288) (#1797)
Update index.md (#2288) (#1797)

* Update index.md

It wasn't at first clear to me you should declare the component before instantiating your app. This hopefully makes that clear to even beginners.

* Update src/v2/guide/index.md

Co-Authored-By: Phan An <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@phanan.net>
vuejs/jp.vuejs.org,vuejs/jp.vuejs.org