fix: outdated cors initializer docs

Author: stlucasgarcia

guides/source/security.md

@@ -1361,7 +1361,7 @@ Next, add an initializer to configure the middleware:
 
 ```ruby
 # config/initializers/cors.rb
-Rails.application.config.middleware.insert_before 0, "Rack::Cors" do
+Rails.application.config.middleware.insert_before 0, Rack::Cors do
   allow do
     origins 'example.com'