minor doc tweak to use mp.spawn in example (#30381)
Summary:
Per pietern's comment in https://github.com/pytorch/pytorch/issues/30022, we can make this example launcher a bit simpler by using `torch.multiprocessing`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30381
Differential Revision: D19292080
Pulled By: rohan-varma
fbshipit-source-id: 018ace945601166ef3af05d8c3e69d900bd77c3b
