Update logging.md

Author: msathieu

docs/general/logging.md

@@ -16,5 +16,6 @@ ActiveModelSerializers.logger = Logger.new(STDOUT)
 You can also disable the logger, just put this in `config/initializers/active_model_serializers.rb`:
 
 ```ruby
+require 'active_model_serializers'
 ActiveSupport::Notifications.unsubscribe(ActiveModelSerializers::Logging::RENDER_EVENT)
 ```