Docs: Update URL of API Documents

Directory structure of TF-M documents has been changed.
The URL needs to be updated.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Ife8a58a841521e68d275a0974a66e71081a01fd8
diff --git a/docs/index.rst b/docs/index.rst
index 1318d1a..2199929 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -60,7 +60,7 @@
             <p>List of supported boards and platforms.</p>
         </li>
         <li class="grid-item">
-            <a href="https://ci.trustedfirmware.org/job/tf-m-build-docs/lastSuccessfulBuild/artifact/trusted-firmware-m/build/install/doc/reference_manual/html/index.html">
+            <a href="https://ci.trustedfirmware.org/view/TF-M/job/tf-m-build-docs-nightly/lastSuccessfulBuild/artifact/trusted-firmware-m/build/docs/reference_manual/html/index.html">
                 <img alt="" src="_static/images/tfm.png"/>
                 <h2>API</h2>
             </a>