pipeline_wan_i2v.py: minor discrepancy between arg default and docstring

### Describe the bug

https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/wan/pipeline_wan_i2v.py

Line 447 (arg default):
```output_type: Optional[str] = "np",```

Line 496 (docstring):
```output_type (`str`, *optional*, defaults to `"pil"`):```

### Reproduction

n/a

### Logs

```shell

```

### System Info

n/a

### Who can help?

_No response_