Fix link to stacks on the concepts overview page (#16653)

Author: julienp

content/docs/iac/concepts/_index.md

@@ -96,7 +96,7 @@ The following topics provide more details on the core concepts of Pulumi and how
         <p>Learn how Pulumi projects are organized and configured.</p>
     </div>
     <div class="md:w-1/2 border-solid border-t-2 border-gray-200">
-        <h3 class="no-anchor pt-4"><a href="/docs/iac/concepts/stack"><i class="fas fa-layer-group pr-2"></i>Stacks</a></h3>
+        <h3 class="no-anchor pt-4"><a href="/docs/iac/concepts/stacks"><i class="fas fa-layer-group pr-2"></i>Stacks</a></h3>
         <p>Learn how to create and deploy stacks.</p>
     </div>
 </div>