Fix syntaxWarning: invalid escape sequence '\s'

[DamonFool]

This fixes the following warnings after https://github.com/vllm-project/vllm/pull/16422 .

/usr/local/lib/python3.12/dist-packages/vllm/utils.py:1236: SyntaxWarning: invalid escape sequence '\s'
  single_newline = re.compile("(?<!\n)\n(?!\n)\s*")
/usr/local/lib/python3.12/dist-packages/vllm/utils.py:1237: SyntaxWarning: invalid escape sequence '\s'
  multiple_newlines = re.compile("\n{2,}\s*")

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[DamonFool]

Thanks @hmellor for the review and help.