update the INSTALL button link (#1560)

Author: annastuchlik

sphinx_scylladb_theme/header.html

@@ -89,7 +89,7 @@
             </li>
         </ul>
         <div class="header-button">
-            <a href="https://docs.scylladb.com/manual/stable/getting-started/installation-common/scylla-web-installer.html" class="button"><i class="icon-download me-2"></i> Install</a>
+            <a href="https://docs.scylladb.com/manual/stable/getting-started/install-scylla/index.html" class="button"><i class="icon-download me-2"></i> Install</a>
         </div>
     </div>
     <div class="header-search-box">