Update return type in findByDatasetId method

- Change return type from `DashboardDrawerList` to `DashboardDrawer`
- Ensure consistency with the `dashboardDrawerService.findByDatasetId` method output

src/main/java/edu/harvard/dbmi/avillach/dictionary/dashboarddrawer/DashboardDrawerController.java

@@ -18,7 +18,7 @@ public ResponseEntity<DashboardDrawerList> findAll() {
     }
 
     @GetMapping("/{id}")
-    public ResponseEntity<DashboardDrawerList> findByDatasetId(@PathVariable Integer id) {
+    public ResponseEntity<DashboardDrawer> findByDatasetId(@PathVariable Integer id) {
         return ResponseEntity.ok(dashboardDrawerService.findByDatasetId(id));
     }
 }