changed spurious heading to paragraph

* it had an inconsistent level with the section's actual heading
* it contained autorefs that didn't render in the TOC

also, added class="informative" to the subsection

Author: pchampin

index.html

@@ -12765,8 +12765,8 @@ <h3>Internationalization Considerations</h3>
 </section>
 
 <section class="appendix informative"><h2>Image Descriptions</h2>
-  <section id="fig-linked-data-graph-descr"><h3>Linked Data Dataset</h3>
-      <h2>Description of the <a href="#fig-linked-data-graph">Linked Data Dataset figure</a> in <a href="#data-model" class="sectionRef"></a></h2>
+  <section id="fig-linked-data-graph-descr" class="informative"><h3>Linked Data Dataset</h3>
+      <p>This secrion describes the <a href="#fig-linked-data-graph">Linked Data Dataset figure</a> in <a href="#data-model" class="sectionRef"></a>.</p>
       <p>The image consists of three dashed boxes, each describing a different
         <a>linked data graph</a>. Each box consists of shapes linked with arrows describing
         the linked data relationships.</p>