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.

open-data-resources.html

@@ -59,15 +59,15 @@ title: Open data resources
           <div class="trigger-tooltip">Endpoint:&nbsp;<a href="{{facility.odr.oai-pmh-endpoint.link}}">link</a>
               [<a href="{{facility.odr.oai-pmh-endpoint.link}}?verb=Identify">Identify</a>]<br/>{%
             if facility.odr.oai-pmh-endpoint.items
-              %}Datasets:&nbsp;<b>{{facility.odr.oai-pmh-endpoint.items.count | thousands_separated}}</b>{%
+              %}Items:&nbsp;<b>{{facility.odr.oai-pmh-endpoint.items.count | thousands_separated}}</b>{%
               assign total_oai_pmh_dataset_count = total_oai_pmh_dataset_count | plus: facility.odr.oai-pmh-endpoint.items.count %}{%
               if facility.odr.oai-pmh-endpoint.items.sets%}
                 <br/>
-                <span class="caret">Categories:&nbsp;{{facility.odr.oai-pmh-endpoint.items.sets.count | thousands_separated}}</span>
+                <span class="caret">Sets:&nbsp;{{facility.odr.oai-pmh-endpoint.items.sets.count | thousands_separated}}</span>
                 <div class="toggled">
                   <table>
                     <tr>
-                      <th>Category</th><th>Datasets</th>
+                      <th>Set</th><th>Items</th>
                     </tr>
                     {% for set in facility.odr.oai-pmh-endpoint.items.sets.details %}
                       <tr>