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