Docs: Update broken URLs

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I9649113504e174d0f1e8d2b59eb731d0d26c87f6
diff --git a/docs/index.rst b/docs/index.rst
index 294e94b..c606c79 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -43,7 +43,7 @@
             <p>Guidelines for integration with TF-M.</p>
         </li>
         <li class="grid-item">
-            <a href="technical_references/index.html">
+            <a href="technical_references/design_docs/index.html">
                 <img alt="" src="_static/images/tfm-reference.png"/>
                 <h2>References</h2>
             </a>