[Doc] cleanup deprecated flag for doc (#18715)

Signed-off-by: calvin chen <120380290@qq.com>

benchmarks/README.md

@@ -146,10 +146,9 @@ python3 vllm/benchmarks/benchmark_serving.py \
 
 ``` bash
 VLLM_USE_V1=1 vllm serve meta-llama/Meta-Llama-3-8B-Instruct \
-    --speculative-model "[ngram]" \
     --ngram_prompt_lookup_min 2 \
     --ngram-prompt-lookup-max 5 \
-    --num_speculative_tokens 5
+    --speculative_config '{"model": "[ngram]", "num_speculative_tokens": 5}
 ```
 
 ``` bash
@@ -274,10 +273,9 @@ python3 vllm/benchmarks/benchmark_throughput.py \
     --output-len=100 \
     --num-prompts=2048 \
     --async-engine \
-    --speculative-model="[ngram]" \
     --ngram_prompt_lookup_min=2 \
     --ngram-prompt-lookup-max=5 \
-    --num_speculative_tokens=5
+    --speculative_config '{"model": "[ngram]", "num_speculative_tokens": 5}
 ```
 
 ```