[Docs]: AntV Dagre 默认 ranker 与文档疑似不一致

### 📍 Which page are you reading?

https://g6.antv.antgroup.com/manual/layout/antv-dagre-layout

### 💭 What's on your mind?

Example needs fixing / 示例需要修复

### 🤔 Tell us more

在线文档中对于 AntV Dagre 默认 ranker 的定义是 `network-simplex`

但是在 @antv/layout 里对于 AntV Dagre 默认 ranker 参数的定义是 `tight-tree`

https://github.com/antvis/layout/blob/6f5c900377033161acdbdc3cb9d769f1063fecf9/packages/layout/src/antv-dagre.ts#L209

### 💡 Got any suggestions?

不知道这种差异是不是因为文档更新不及时导致的，我不太清楚在这种情况下到底默认是用 `network-simplex` 还是 `tight-tree`

Start with the AntV Dagre layout page linked in the issue and compare its documented default ranker with the definition in packages/layout/src/antv-dagre.ts around line 209. Confirm which value is the actual default, then update the documentation so it matches the implementation and verify the example or parameter description is consistent.

Written by the indexing model from the issue text.