Fix TOC link path

Hi.
This PR fixes link paths in TOC.md and add some spec.

We sometimes use sub-directories in request spec dir.
For example, like this:
```
spec/
 requests/
   api/
     entries_spec.rb
     ...
```

And  Autodoc outputs files like below:
```
doc/
  toc.md
  admin/
    entries.md
```

But `toc.md` link path is broken:
```
## Table of Contents
* [entries.md](entries.md)
 * [GET /admin/entries](entries.md#get-adminentries)
```

My expected output is below:
```
## Table of Contents
* [admin/entries.md](admin/entries.md)
 * [GET /admin/entries](entries.md#get-adminentries)
```