use async method over asyn getter to retrieve sub learner-groups.

fixes a minor bug that only got unearthed when shuffling code around.

Author: stopfstedt

packages/frontend/app/components/learner-group/calendar.gjs

@@ -53,7 +53,7 @@ export default class LearnerGroupCalendarComponent extends Component {
   async loadEvents(learnerGroup, showSubgroupEvents) {
     let learnerGroupOfferings = await this.getEventsFromLearnerGroups([learnerGroup], false);
     if (showSubgroupEvents) {
-      const subLearnerGroups = await learnerGroup.get('allDescendants');
+      const subLearnerGroups = await learnerGroup.getAllDescendants();
       const subLearnerGroupOfferings = await this.getEventsFromLearnerGroups(
         subLearnerGroups,
         true,