doc(ja) typo in variable names (#1894)

* Typo fixes

There are a few wrong variables.

* Update named-routes.md

Author: miyanokomiya

docs/ja/essentials/named-routes.md

@@ -6,7 +6,7 @@
 const router = new VueRouter({
   routes: [
     {
-      path: '/user/:id',
+      path: '/user/:userId',
       name: 'user',
       component: User
     }