Update "Layouts and Rendering in Rails" guide [ci skip]

- Remove mention about `ActionView::TemplateHandlers` since this
    module was removed by c1304098cc.
    Change word `subclasses` to `nested classes`.
    See c7408a0e40
  - Remove useless sentence "Beginning with Rails 2, the standard extensions
    are `.erb` for ERB (HTML with embedded Ruby), and `.builder` for Builder (XML generator)."

guides/source/layouts_and_rendering.md

@@ -97,7 +97,7 @@ If we want to display the properties of all the books in our view, we can do so
 <%= link_to "New book", new_book_path %>
 ```
 
-NOTE: The actual rendering is done by subclasses of `ActionView::TemplateHandlers`. This guide does not dig into that process, but it's important to know that the file extension on your view controls the choice of template handler. Beginning with Rails 2, the standard extensions are `.erb` for ERB (HTML with embedded Ruby), and `.builder` for Builder (XML generator).
+NOTE: The actual rendering is done by nested classes of the module [`ActionView::Template::Handlers`](http://api.rubyonrails.org/classes/ActionView/Template/Handlers.html). This guide does not dig into that process, but it's important to know that the file extension on your view controls the choice of template handler.
 
 ### Using `render`