add missing verb, and follow @hartig's proposal

Author: pchampin

spec/index.html

@@ -297,13 +297,13 @@ <h3>RDF Vocabularies and Namespace IRIs</h3>
     </table>
 
     <p>In some serialization formats it is common
-      to some [=namespace IRIs=] to arbitrary [=namespace prefixes=],
+      to associate some [=namespace IRIs=] to arbitrary [=namespace prefixes=],
       and to abbreviate <a>IRIs</a> that start with one of those <a>namespace IRIs</a> by using the corresponding <a>namespace prefix</a>,
       in order to assist readability.
-      For example, the IRI
-      <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral</code>
-      would be abbreviated as <code>rdf:XMLLiteral</code>
-      with the prefix mapping from <a href="#tab-vocab-ns"></a>.
+      For example,
+      based on the prefix mapping in <a href="#tab-vocab-ns"></a>,
+      the IRI <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral</code>
+      would be abbreviated as <code>rdf:XMLLiteral</code>.
       Note however that these abbreviations are <em>not</em> valid IRIs (or only coincidentally),
       and must not be used in contexts where IRIs are expected.
       Namespace IRIs and namespace prefixes are <em>not</em> a formal part of the