Delete the line 'npm install' (#1475)

This is already done by the command 'vue init ..' which installs the node_modules/

Author: pranayaryal

src/v2/guide/installation.md

@@ -67,7 +67,6 @@ $ npm install --global vue-cli
 $ vue init webpack my-project
 # install dependencies and go!
 $ cd my-project
-$ npm install
 $ npm run dev
 ```