Removed confusing statements about routes in getting started guide [ci skip]

- routes for CRUD already exists as `resources :posts` is in
  `config/routes.rb`
- So we don't have to add any new route in the `config/routes.rb`
- As per #11644, the adding of routes which are already there confuses user, so here its
  changed to referring output of `rake routes`
Merge pull request #11666 from prathamesh-sonpatki/doc-fix

Removed confusing statement about adding route for show action in getting started guide [ci skip]