[Docs] Information about ActionDispatch::DebugExceptions in rake task

railties/guides/source/rails_on_rack.textile

@@ -95,6 +95,7 @@ use ActiveSupport::Cache::Strategy::LocalCache
 use Rack::Runtime
 use Rails::Rack::Logger
 use ActionDispatch::ShowExceptions
+use ActionDispatch::DebugExceptions
 use ActionDispatch::RemoteIp
 use Rack::Sendfile
 use ActionDispatch::Callbacks