Add missing translation variable

ar/backend.php

@@ -197,3 +197,4 @@ $backend_tl['display_topic_startpage'] = 'Display topic on startpage';
 $backend_tl['hide_topic_startpage'] = 'Hide topic on startpage';
 $backend_tl['startpage_normal'] = 'normal';
 $backend_tl['startpage_scrollitelling'] = 'Scrollitelling';
+$backend_tl['enable_direct_object_links'] = 'Link objects directly in md';