luci-app-keepalived: fix copy past error for the debug option

The 'debug' configuration option in the LuCI incorrectly overwrites the
'preempt_delay' configuration option due to a copy past error. Using the
correct uci configuration option 'debug' fixes this error.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

applications/luci-app-keepalived/htdocs/luci-static/resources/view/keepalived/vrrp_instance.js

@@ -218,7 +218,7 @@ return view.extend({
 		o.placeholder = '300';
 		o.modalonly = true;
 
-		o = s.taboption('advanced', form.ListValue, 'preempt_delay', _('Debug'),
+		o = s.taboption('advanced', form.ListValue, 'debug', _('Debug'),
 			_('Debug Level'));
 		o.default = '0';
 		o.value('0');