[ci skip] Add missing setting locale code snippet.

This is for 3 Internationalizing your Application of i18n.md.

Missing the code as mentioned in 2.3, setting locale.

Without this, the example to add config/locales/pirate.yml will not work.

guides/source/i18n.md

@@ -315,6 +315,17 @@ Yourapp::Application.routes.draw do
 end
 ```
 
+```ruby
+# app/controllers/application_controller.rb
+class ApplicationController < ActionController::Base
+  before_action :set_locale
+
+  def set_locale
+    I18n.locale = params[:locale] || I18n.default_locale
+  end
+end
+```
+
 ```ruby
 # app/controllers/home_controller.rb
 class HomeController < ApplicationController