[bugfix] small fix logic issue (#18999)

Signed-off-by: reidliu41 <reid201711@gmail.com>
Co-authored-by: reidliu41 <reid201711@gmail.com>

vllm/engine/arg_utils.py

@@ -455,7 +455,7 @@ class EngineArgs:
             title="ModelConfig",
             description=ModelConfig.__doc__,
         )
-        if 'serve' not in sys.argv[1:] and '--help' not in sys.argv[1:]:
+        if not ('serve' in sys.argv[1:] and '--help' in sys.argv[1:]):
             model_group.add_argument("--model", **model_kwargs["model"])
         model_group.add_argument("--task", **model_kwargs["task"])
         model_group.add_argument("--tokenizer", **model_kwargs["tokenizer"])