Rename `PhutilBufferedIteratorExample` to `PhutilExampleBufferedIterator`

Summary: Ref T5655. `PhutilExampleBufferedIterator` seems clearer than `PhutilBufferedIteratorExample`.

Test Plan: Ran `arc unit`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9995