Sorry about that, it should be noted in the tutorial text that this has been changed.
The Prime content was a smaller package based on the old categories of Prime, Broadcast, Visualize, Studio, etc.
You can download all, as there are more HDRIs available. (After download). With the magnifying glass, you can get all at once: see image below. Try also the exr suffix. Typically it indicates better quality, if the content was never limited before that, like being stored as radiance.
However, if you are looking for HDR images and nothing else, please have a look here:
Please note that the radiance (dot-hdr) format, is suitable for illumination, but has a horrible color precision, so it should not be used in high-end work. For exploring the techniques, they are just fine.
(Link below, in my signature) I have a more extended series about HDR and how to create those in production-level quality.
All the best
Click thumbnail to see full-size image