docs: Corrected reference link for NotificationManager.notify().

The end of the first paragraph of the "Creating a Notification"
section (within the "Notifications" page) now links to the correct
version of notify() -- i.e. the one within the NotificationManager
class.

Bug: 27708810
Change-Id: I7cc9608f646518bf5f7fc3f4a231c9dece693363