[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/developer_guide/performance_and_debug/msprobe_guide.md

@@ -112,7 +112,7 @@ JSON
      -d '{
            "model": "Qwen/Qwen2.5-0.5B-Instruct",
            "prompt": "Explain gravity in one sentence.",
-           "max_tokens": 32,
+           "max_completion_tokens": 32,
            "temperature": 0
          }' | python -m json.tool
    ```

docs/source/developer_guide/performance_and_debug/service_profiling_guide.md

@@ -50,7 +50,7 @@ curl http://localhost:8000/v1/completions \
     -d '{
          "model": "Qwen/Qwen2.5-0.5B-Instruct",
         "prompt": "Beijing is a",
-        "max_tokens": 5,
+        "max_completion_tokens": 5,
         "temperature": 0
 }' | python3 -m json.tool
 ```