Fix "searchUsers" usage in Group_LDAP.php

`Access::searchUsers` expects an array as the second parameter, `Group_LDAP.php` called the method with a simple string, resulting in an exception and preventing folders shared with dynamic groups from working. This trivial change fixes the usage in Group_LDAP.

Fixes #25163

Signed-off-by: Henning Hoefer <hho@users.noreply.github.com>