adhere to rubocop guidelines

config/environments/development.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
 
@@ -18,7 +20,7 @@ Rails.application.configure do
 
     config.cache_store = :memory_store
     config.public_file_server.headers = {
-      'Cache-Control' => 'public, max-age=172800'
+      'Cache-Control' => 'public, max-age=172800',
     }
   else
     config.action_controller.perform_caching = false