Setting up a Public REST API

You are using the wrong URL I think

Try this: https://chale1-developer-edition.na14.force.com/test/services/apexrest/helloWorld

EDIT - When I looked at the HTML that came back original URL in Postman, I saw this message:

https://chale1-developer-edition.na14.force.com/test</i> is down for maintenance

So I realized he must have used added a suffix to the Default Web Address in the Site called test. The REST URL includes the default suffix as part of the full URL, so I added that the URL he provided, and it worked...