Rename PhabricatorDestructableInterface interface

Summary: Ref T5655. The PhabricatorDestructibleInterface interface is misspelled as PhabricatorDestructableInterface. Fix the spelling mistake.

Test Plan: grep. Seeing as this interface is fairly recent, I don't expect that this would cause any widespread breakages.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9988