Improve-mm-and-pooler-and-decoding-configs (#16789)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/source/models/supported_models.md

@@ -788,7 +788,7 @@ llm = LLM(
 Online serving:
 
 ```bash
-vllm serve Qwen/Qwen2-VL-7B-Instruct --limit-mm-per-prompt image=4
+vllm serve Qwen/Qwen2-VL-7B-Instruct --limit-mm-per-prompt '{"image":4}'
 ```
 
 **This is no longer required if you are using vLLM V1.**