[doc] trivial changes to keep same path dir in doc for setup-dev.py

[xingyu-long]

Why are these changes needed?

my venv was removed accidentally and then when re-setup my environment, I found this trivial doc issue.

same as title, we probably should keep same path dir in doc for setup-dev.py

Related issue number

NA

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[xingyu-long]

@richardliaw is it possible you can help to merge it in? since I am not authorized to push. thanks!