[core] Fix doc for User-Spawned Process

[ruisearch42]

Why are these changes needed?

The doc was not accurate.

Related issue number

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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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