Fix prop change handler for guild challenges. (#14169)

Fix props' change handler which is called when guild is changed from
notification.

Tests:

+ Guild challenges updated successfully when guild is changed from
notification center.

Co-authored-by: neel <neel@helpshift.com>

website/client/src/components/challenges/groupChallenges.vue

@@ -112,12 +112,10 @@ export default {
     },
   },
   watch: {
-    'group._id': {
+    'group._id': function groupId () {
-      async groupId () {
       this.loadChallenges();
     },
   },
-  },
   mounted () {
     this.loadChallenges();
   },