Fix validation command in README

[xuyangwang0825]

I encountered the following error when running the validation command:

usage: vllm_validate_poly.py [-h] --save_dir SAVE_DIR --decode_model_name
                             DECODE_MODEL_NAME --validate_model_name
                             VALIDATE_MODEL_NAME [--tp TP]
                             [--max_decode_tokens MAX_DECODE_TOKENS]
                             [--decode_batching_size DECODE_BATCHING_SIZE]
                             [--dtype DTYPE] [--attn ATTN]
                             [--scale_decode_mantissa SCALE_DECODE_MANTISSA]
vllm_validate_poly.py: error: unrecognized arguments: --n_samples 4 --dataset_name stingning/ultrachat

It looks like --n_samples and --dataset_name are not supported arguments in vllm_validate_poly.py. Also, the --save_dir argument should be set to signatures where we generated results in the previous step.