fix(docs): Update index page (#1008)

Updated the index page to link to 6.1.x instead of 6.0.x

Author: matrei

plugin/src/docs/index.tmpl

@@ -84,7 +84,7 @@ img {
 
 		<h2>Grails 6.x.x and beyond</h2>
         <ul>
-            <li><a href="/grails-spring-security-core/6.0.x/index.html">User guide</a></li>
+            <li><a href="/grails-spring-security-core/6.1.x/index.html">User guide</a></li>
         </ul>
 
 		<h2>Grails 5.x.x and beyond</h2>