refs #22480 w3c html validate: add missing quotion mark

res/theme/templates/search/controls/grouping.phtml

@@ -2,7 +2,7 @@
 
 if ($this->results instanceof Results): ?>
     <form class="form-inline search-grouping" action="<?= $this->currentPath() ?>"
-          method="POST" id=grouping">
+          method="POST" id="grouping">
         <div class="form-group">
             <label for="grouping-checkbox"><?= $this->transEsc('group hits') ?></label>
             <input type="checkbox" name="grouping-enabled" value=""