Fix a defunct link to Shape Trees specification

Author: mrkvon

index.html

@@ -150,7 +150,7 @@ <h2 property="schema:name">Work Items</h2>
                     <td><time datetime="2021-06-30">2021-06-30</time></td>
                   </tr>
                   <tr>
-                    <td><a href="https://shapetrees.github.io/specification/spec" rel="cito:citesForInformation">Shape Trees</a></td>
+                    <td><a href="https://shapetrees.org/TR/specification/" rel="cito:citesForInformation">Shape Trees</a></td>
                     <td><a href="https://github.com/shapetrees/specification">https://github.com/shapetrees/specification</a></td>
                     <td>Editor’s Draft</td>
                     <td>Ecosystem Proposal</td>