Fix rubocop error

Author: dsandstrom

config/environments/development.rb

@@ -71,5 +71,5 @@
   config.action_mailer.perform_deliveries = true
   config.action_mailer.delivery_method = :letter_opener
 
-  config.action_mailer.preview_path = "#{Rails.root}/spec/mailers/previews"
+  config.action_mailer.preview_path = Rails.root.join('spec/mailers/previews')
 end