Link references are not correctly validated.

Found when run against dotnet/core repo:

```md
See the [Release Notes](release-notes) for details about what is included in this update.

[release-notes]: https://github.com/dotnet/core/blob/main/release-notes/3.0/3.0.0-preview4-27615-11/3.0.0-preview4-27615-11.md
```