Update index.html

Author: MarouaneBenbrahim

website/index.html

@@ -499,7 +499,7 @@ <h3>Conference Paper</h3>
                         </p>
                     </div>
                     <div class="paper-buttons">
-                        <a href="paper.pdf" class="btn btn-outline" target="_blank">
+                        <a href="https://github.com/XGraph-Team/SumoXPypsa/blob/main/website/paper.pdf" class="btn btn-outline" target="_blank">
                             <i class="fas fa-file-pdf"></i>
                             PDF
                         </a>