Update PyTorch Lightning DDP Documentation Links in README.md

[amorehead]

What does this PR do?

Updates PyTorch Lightning DDP Documentation Links in README.md. Otherwise, the current links will lead to 404 webpage errors.

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
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[amorehead]

@ashleve, feel free to merge this one in at your convenience.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (271972c) 83.75% compared to head (4804d82) 83.75%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #601   +/-   ##
=======================================
  Coverage   83.75%   83.75%           
=======================================
  Files          11       11           
  Lines         357      357           
=======================================
  Hits          299      299           
  Misses         58       58           

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[ashleve]

Thanks!