[Misc] Fix path and python alias errors in disagg_prefill exmaples

[Jeffwan]

1. this file has been moved to other subfolder but the relative path is not updated, results in failure.
   

2. change python -> python3. this file has both python and python3 in use. I just change to python3 for consistency. some system like vllm official container doesn't have python alias.

root@decb0cd0b18e:/vllm-workspace/# python
bash: python: command not found

[github-actions]

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