Fix bad path in prometheus example (vllm-project#12481)

Signed-off-by: mgoin <[email protected]>

Author: mgoin

examples/online_serving/prometheus_grafana/README.md

@@ -24,7 +24,7 @@ Submit some sample requests to the server:
 ```bash
 wget https://huggingface.co/datasets/anon8231489123/ShareGPT_Vicuna_unfiltered/resolve/main/ShareGPT_V3_unfiltered_cleaned_split.json
 
-python3 ../../benchmarks/benchmark_serving.py \
+python3 ../../../benchmarks/benchmark_serving.py \
     --model mistralai/Mistral-7B-v0.1 \
     --tokenizer mistralai/Mistral-7B-v0.1 \
     --endpoint /v1/completions \