updated form.phtml

Fixed #19780
Incorrect class name on Orders and returns page.

Author: Shikha Mishra

app/code/Magento/Sales/view/frontend/templates/guest/form.phtml

@@ -10,7 +10,7 @@
 <form class="form form-orders-search" id="oar-widget-orders-and-returns-form" data-mage-init='{"ordersReturns":{}, "validation":{}}' action="<?= /* @escapeNotVerified */ $block->getActionUrl() ?>"
       method="post" name="guest_post">
     <fieldset class="fieldset">
-        <legend class="admin__legend"><span><?= /* @escapeNotVerified */ __('Order Information') ?></span></legend>
+        <legend class="legend"><span><?= /* @escapeNotVerified */ __('Order Information') ?></span></legend>
         <br>
 
         <div class="field id required">