Fix broken URL link in ddp_tutorial.rst

[Choumingzhao]

Simple correct to broken URL tag which may cause by typo:
torch elastic/torchrun<https://pytorch.org/docs/stable/elastic/quickstart.html>__

cc @svekars @brycebortree @sekyondaMeta @carljparker

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[svekars]

Please sign the CLA so we can merge this PR. Thanks!

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