update en/application-dev/notification/Readme-EN.md.

Signed-off-by: Nzengyawen <zengyawen1@huawei.com>

en/application-dev/notification/Readme-EN.md

 # Common Event and Notification
 
 - [Common Event and Notification Overview](notification-brief.md)
-- Common Event
-  - [Common Event Development](common-event.md)
-- Notification
-  - [Notification Development](notification.md)
-- Debugging Tools
-  - [Debugging Assistant Usage](assistant-guidelines.md)
+- [Common Event Development](common-event.md)
+- [Notification Development](notification.md)
+- [Debugging Assistant Usage](assistant-guidelines.md)