There could be multiple reasons for it not working. Considering you're trying on AEM cloud service though the document you've referred uses older maven archetype, it could be due to that. I hope you're using latest archetype 23/ 24 in this case.
Also, please check if the model is available under /system/console/status-slingmodels and you're able to see log statements added for HelloWorldModel.
You can also refer to the below tutorial for setting up sling model