Fix call to non-existing getDisplayName() in DiffusionRepositoryClusterEngine.php

Summary:
DiffusionRepositoryClusterEngine calls $this->getDisplayName() which is not defined in this class or its parent class.
Looking at the variable use in the companying error message, getting the name of the $repository was intended instead.

Test Plan: Carefully read the code and check available methods in its parent classes.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25653