[TASK][REFACTOR] fix errors in 01 and 02 guides

Author: Nils Henning

docs/guides/essential/01_setup.md

@@ -272,6 +272,7 @@ class Demo::App < Matestack::Ui::App
     nav do
       link path: root_path, text: 'First Page'
       link path: second_page_path, text: 'Second Page'
+    end
     header do
       heading size: 1, text: 'Demo App'
     end

docs/guides/essential/02_active_record.md

@@ -77,10 +77,11 @@ Rails.application.routes.draw do
 end
 ```
 
-After that we create the corresponding person controller in `app/controllers/person_controller.rb` which will handle the index action. The index action should render the persons index page which we will create afterwards.
+After that we create the corresponding person controller in `app/controllers/person_controller.rb` which will handle the index action. The index action should render the persons index page which we will create afterwards. Do not forget to define the `matestack_app` which should be used as a layout.
 
 ```ruby
 class PersonsController < ApplicationController
+  matestack_app Demo::App
 
   def index
     render Demo::Pages::Persons::Index