fix up references to ground

Author: pfps

spec/index.html

@@ -301,7 +301,7 @@ <h2>Notation and Terminology</h2>
       _:x ex:p _:x .
     </pre>
 
-    <p>is lean. <a>Ground</a> graphs are lean. </p>
+    <p>is lean. A <a>ground RDF graph</a> is lean. </p>
 
   <section id="unions_merges">
     <h3>Shared blank nodes, unions and merges</h3>
@@ -655,7 +655,7 @@ <h3>Properties of simple entailment and satisfiability (Informative)</h3>
       one mathematical graph is a subgraph of another) as detecting simple entailment between RDF graphs.
       This construction (due to Jeremy Carroll) uses graphs all of whose nodes are blank nodes.
       The complexity of checking simple entailment is reduced by having fewer blank nodes in the conclusion E.
-      When E is a <a>ground</a> graph, it is simply a matter of checking the subset relationship on sets of triples.</p>
+      When E is a <a>ground RDF graph</a>, it is simply a matter of checking the subset relationship on sets of triples.</p>
 
     <p><a>Interpolation</a> has a number of direct consequences, for example:</p>