Fix security URLs prefix.

This appears to be an unused prefix with a typo to an endpoint that
doesn't exist.

Author: davidlehn

vocab/credentials/credentials.html

@@ -144,7 +144,7 @@ <h2>Introduction</h2>
         <dt><code>dc</code>:</dt>
         <dd><code>http://purl.org/dc/terms/</code></dd>
         <dt><code>sec</code>:</dt>
-        <dd><code>https://w3.org/2018/security#</code></dd>
+        <dd><code>https://w3id.org/security#</code></dd>
         <dt><code>xsd</code>:</dt>
         <dd><code>http://www.w3.org/2001/XMLSchema#</code></dd>
       </dl>

vocab/credentials/template.html

@@ -152,7 +152,7 @@ <h2>Introduction</h2>
         <dt><code>dc</code>:</dt>
         <dd><code>http://purl.org/dc/terms/</code></dd>
         <dt><code>sec</code>:</dt>
-        <dd><code>https://w3.org/2018/security#</code></dd>
+        <dd><code>https://w3id.org/security#</code></dd>
         <dt><code>xsd</code>:</dt>
         <dd><code>http://www.w3.org/2001/XMLSchema#</code></dd>
       </dl>