There is definitely an extra } there, try this:
(function executeRule(current, previous /* previous state */) {
var incidentGR = new GlideRecord('incident');
if (incidentGR.get('rfc', current.sys_id)) {
// Update incident work notes
incidentGR.work_notes = 'Change request has been rejected: ' + current.number;
// Here you can also add a status change maybe
incidentGR.update();
}
})(current, previous);
