Apply suggested fix to tests/GitAttributes/ExistenceCheckerTest.php from Copilot Autofix (#96)

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

Author: coisa

tests/GitAttributes/ExistenceCheckerTest.php

@@ -33,7 +33,7 @@ final class ExistenceCheckerTest extends TestCase
     use ProphecyTrait;
 
     /**
-     * @property ObjectProphecy<FilesystemInterface> $filesystem
+     * @var ObjectProphecy<FilesystemInterface>
      */
     private readonly ObjectProphecy $filesystem;