Broken link to md_flexible example

In the [home page of AutoPAS](https://autopas.github.io/doxygen_documentation/git-master/), the link to the md-flexible example is broken: https://autopas.github.io/doxygen_documentation/git-master/examples/md-flexible

I would have fixed it already, but I don't really understand where this page draws its content from.

首先检查 AutoPas.github.io 仓库和链接的 AutoPAS 主页，以找到生成页面内容的源代码。跟踪 md-flexible 示例链接，并更新其源代码，使该链接能够在文档中记载的 URL 上成功解析。

由索引模型根据 Issue 内容生成。