Clean up Javadocs for UEventObserver

This commit removes a couple of apostrophes from the word "UEvents" as
the apostrophes would only be needed if something that belongs to the
UEvent was being talked about. Instead, the UEvents are being talked
about themselves - as a bunch.

Change-Id: I6a7908c6b73c0739102b632d2499e0f1e3c2d47f