fix($demo): 404 page active link of sidebar (#2021)

pages/examples/404.html

@@ -413,13 +413,13 @@
             </a>
             <ul class="nav nav-treeview">
               <li class="nav-item">
-                <a href="../examples/404.html" class="nav-link">
+                <a href="../examples/404.html" class="nav-link active">
                   <i class="fa fa-circle-o nav-icon"></i>
                   <p>Error 404</p>
                 </a>
               </li>
               <li class="nav-item">
-                <a href="../examples/500.html" class="nav-link active">
+                <a href="../examples/500.html" class="nav-link">
                   <i class="fa fa-circle-o nav-icon"></i>
                   <p>Error 500</p>
                 </a>