Use be_successful instead of be_success

Rails 5.2 warns:

> The `success?` predicate is deprecated and will be removed in Rails 6.0.
> Please use `successful?` as provided by Rack::Response::Helpers.

RSpec correspondingly provides `be_successful`.

spec/controllers/schedules_controller_spec.rb

@@ -23,7 +23,7 @@ describe SchedulesController do
       end
 
       it 'has 200 status code' do
-        expect(response).to be_success
+        expect(response).to be_successful
       end
     end
   end