Fix action rename example in controllers guides

Only action name should be changed to match route renaming.
Change of passing template's name leads to bug, as the template is not renamed.
And description above clearly talks only about action.

guides/controllers.md

@@ -47,7 +47,7 @@ defmodule HelloWeb.PageController do
  ...
 
  def index(conn, _params) do
- render(conn, :index)
+ render(conn, :home)
  end
 end
 ```