Correct a method signature in DifferentialDraftField

Summary:
Ref T12190. See https://discourse.phabricator-community.org/t/exception-preventing-access-to-differential-application/606.

(I have a followup to fix the root issue.)

Test Plan: Loaded Differential with an eye on the error log in PHP7, no longer saw warnings.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12190

Differential Revision: https://secure.phabricator.com/D18723