Update webpage language to use correct terms from OAI-PMH

Motivation:

It is certainly not guaranteed that all OAI-PMH items are datasets;
indeed, we have a counter-example from HZDR, which also describes
software.

Also, although 'category' (to my thinking) would be a better term,
OAI-PMH uses `set`.  Using a different term is more confusing than using
any better term.

Modification:

Switch from `Category` to `Set` and `Dataset` to `Item`.

Result:

Webpage is easier to underxtand and less likely to cause confusion.