[readme] 'huggingface-cli download' is deprecated

[LeiYangGH]

readme instruction:

huggingface-cli download microsoft/BitNet-b1.58-2B-4T-gguf --local-dir models/BitNet-b1.58-2B-4T

⚠️ Warning: 'huggingface-cli download' is deprecated. Use 'hf download' instead.