test: update test_mount_ostree_error

The previous commits changed the error message which is part of
this test. Update it accordingly.

Author: mvo5

test/test_manifest.py

@@ -304,7 +304,7 @@ def test_mount_ostree_error(tmpdir_factory, build_container):
             "manifest", f"{container_ref}",
             "--config", "/output/config.json",
         ], stderr=subprocess.PIPE, encoding="utf8")
-    assert 'The following errors occurred while validating custom mountpoints:\npath "/ostree" is not allowed' \
+    assert 'the following errors occurred while validating custom mountpoints:\npath "/ostree" is not allowed' \
         in exc.value.stderr