[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/Qwen2.5-7B.md

@@ -127,7 +127,7 @@ curl http://<IP>:<Port>/v1/completions \
     -d '{
         "model": "qwen-2.5-7b-instruct",
         "prompt": "Beijing is a",
-        "max_tokens": 5,
+        "max_completion_tokens": 5,
         "temperature": 0
     }'
 ```