Fix regression on error-msg matching on functests

The error message comes from pulpcore and contained a typo.
When it was fixed [0], our test failed to match.

Ideally we would want to have Error Codes, but at this point it
would be too costly to implement that.

[0] pulp/pulpcore#5935

pulp_rpm/tests/functional/api/test_domains.py

@@ -161,15 +161,15 @@ def test_object_creation(
     assert e.value.status == 400
     # What key should this error be under? non-field-errors seems wrong
     assert json.loads(e.value.body) == {
-        "non_field_errors": [f"Objects must all be apart of the {domain_name} domain."]
+        "non_field_errors": [f"Objects must all be a part of the {domain_name} domain."]
     }
 
     with pytest.raises(ApiException) as e:
         sync_body = {"remote": default_remote.pulp_href}
         rpm_repository_api.sync(repo.pulp_href, sync_body)
     assert e.value.status == 400
     assert json.loads(e.value.body) == {
-        "non_field_errors": [f"Objects must all be apart of the {domain_name} domain."]
+        "non_field_errors": [f"Objects must all be a part of the {domain_name} domain."]
     }