Changed dynamic segment to userId (#739)

:to="" and push() were using param name "userId" but the route path was using "/user/:id". This is confusing because how would params {userId:123} translate to :id?

Author: dharik

docs/en/essentials/named-routes.md

@@ -6,7 +6,7 @@ Sometimes it is more convenient to identify a route with a name, especially when
 const router = new VueRouter({
   routes: [
     {
-      path: '/user/:id',
+      path: '/user/:userId',
       name: 'user',
       component: User
     }