message: docs: fix RouteMeta typing example
  Without this change the example would have no exported members other than RouteMeta
diff: @@ -56,6 +56,8 @@ router.beforeEach((to, from) => {
  It is possible to type the meta field by extending the `RouteMeta` interface:
  ```ts
  +import 'vue-router'
  +
  declare module 'vue-router' {
  interface RouteMeta {
  // is optional
  