fix leave editor modal

no issue
- following up from 6680, fixes a missing set of parentheses

ghost/admin/app/components/modals/leave-editor.js

@@ -3,7 +3,7 @@ import ModalComponent from 'ghost/components/modals/base';
 export default ModalComponent.extend({
     actions: {
         confirm() {
-            this.get('confirm').finally(() => {
+            this.get('confirm')().finally(() => {
                 this.send('closeModal');
             });
         }