index.html: update summer school link

index.html

@@ -59,7 +59,7 @@ function sitesearch() {
     <li><A HREF="doc/gf-quickstart.html">QuickStart</A>
     <li><A HREF="doc/gf-reference.html">QuickRefCard</A>
     <li><A HREF="doc/gf-shell-reference.html">GF Shell Reference</A>
-    <li><a href="http://school.grammaticalframework.org/2017/"><b>GF Summer School</b></a>
+    <li><a href="http://school.grammaticalframework.org/"><b>GF Summer School</b></a>
   </ul>
   <ul>
     <li><A HREF="gf-book">The GF Book</A>