sidebar not aligned between file and folder classes

[DanielYWoo]

sidebar里带链接的子节点显示都正确, 但是目录节点对不齐。比如这个sandbox里的item 3.

See: https://codesandbox.io/s/falling-paper-w19udv?file=/README.md

- [Item 1](/)
- [Item 2](/)
- Item 3 (not aligned correctly)
  - [Item 3.1](/)
  - [Item 3.2](/)
- [Item 4](/)
- [Item 5](/)