Update part-6-performance-normalization.md

line 480 when the original filter returned notifications where !notification.read was the condition, due to the second render of <NotificationList/> allNotificationsRead is being called in useLayoutEffect and notifications.filter(n=>!n.read) is 0.
So we should filter based upon isNew.

Author: anthonyss09

docs/tutorials/essentials/part-6-performance-normalization.md

@@ -477,7 +477,7 @@ export const Navbar = () => {
   const dispatch = useDispatch()
   // highlight-start
   const notifications = useSelector(selectAllNotifications)
-  const numUnreadNotifications = notifications.filter(n => !n.read).length
+  const numUnreadNotifications = notifications.filter(n => n.isNew).length
   // highlight-end
   // omit component contents
   // highlight-start