[Doc] Update max_tokens to max_completion_tokens in all docs (#6248)

### What this PR does / why we need it?

Fix:

```
DeprecationWarning: max_tokens is deprecated in favor of the max_completion_tokens field.
```

- vLLM version: v0.14.1
- vLLM main:
d68209402d

Signed-off-by: shen-shanshan <467638484@qq.com>

docs/source/tutorials/Qwen2.5-Omni.md

@@ -156,7 +156,7 @@ curl http://127.0.0.1:8000/v1/chat/completions   -H "Content-Type: application/j
         ]
       }
     ],
-    "max_tokens": 100,
+    "max_completion_tokens": 100,
     "temperature": 0.7
   }'