Change `william.cage` to `example`

Feedback in https://github.com/serghey-rodin/vesta/issues/1530

web/templates/admin/add_mail_acc.html

@@ -158,7 +158,7 @@
                         </tr>
                         <tr>
                             <td><?=__('Username')?>:</td>
-                            <td><div><span id="v_account">william.cage</span>@<?=htmlentities(trim($v_domain, "'"))?></div></td>
+                            <td><div><span id="v_account">example</span>@<?=htmlentities(trim($v_domain, "'"))?></div></td>
                         </tr>
                         <tr>
                             <td><?=__('Password')?>:</td>