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

src/applications/diffusion/protocol/DiffusionRepositoryClusterEngine.php

@@ -586,7 +586,7 @@ final class DiffusionRepositoryClusterEngine extends Phobject {
           'for %s seconds(s). This repository will be frozen.',
           $this->clusterWriteOwner,
           $device->getName(),
-          $this->getDisplayName(),
+          $repository->getDisplayName(),
           new PhutilNumber($duration)));
     }