use truthy value for unsubcribing

src/app/courses/courses.component.ts

@@ -332,7 +332,7 @@ export class CoursesComponent implements OnInit, AfterViewInit, OnDestroy {
  }
 
  ngOnDestroy() {
- this.onDestroy$.next(false);
+ this.onDestroy$.next(true);
  this.onDestroy$.unsubscribe();
  }
 }