[Train] Update checkpoint path for RayTrainReportCallbacks.

[woshiyyya]

Why are these changes needed?

In our doc, we didn't explicitly explain how to retrieve the original checkpoint files if using Ray Train provided report callbacks. This PR add some code snippets for users.

Related issue number

Closes #40082

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• 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.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
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  • This PR is not tested :(

[matthewdeng]

Can you try this:

Suggested change

	Checkpoints will be saved in the following structure:
	.. testcode::
	# checkpoint_00000*/ Ray Train Checkpoint
	# └─ checkpoint.ckpt PyTorch Lightning Checkpoint
	Checkpoints will be saved in the following structure::
	checkpoint_00000*/ Ray Train Checkpoint
	└─ checkpoint.ckpt PyTorch Lightning Checkpoint

https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks