Spi adaptive extension document instance code has an error

[http://dubbo.apache.org/zh-cn/docs/source_code_guide/adaptive-extension.html](url)

```java
// 1.从 URL 中获取 WheelMaker 名称
        String wheelMakerName = url.getParameter("Wheel.maker");
        if (name == null) {
            throw new IllegalArgumentException("wheelMakerName == null");
        }
```

the name is error.