fix typo in test_dataloader test_multiprocessing_contexts (#43343)
Summary:
https://github.com/pytorch/pytorch/issues/22990 added a multiprocessing_context argument to DataLoader, but a typo in the test causes the wrong DataLoader class to be used.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43343
Reviewed By: glaringlee
Differential Revision: D23299452
Pulled By: malfet
fbshipit-source-id: 9489c48b83bce36f46d350cad902f7ad96e1eec4
