Fix "show post" link in Getting Started guide

Following the Getting Started guide, this line of code produces an error:

    <td><%= link_to 'Show', post %></td>

The error is:
> ActionController::UrlGenerationError in Posts#index
> No route matches {:action=>"show", :controller=>"posts"} missing required keys: [:id]

It looks like the post method is just missing the necessary argument, which I've added in this pull request.