The Github wikis are backed by a separate git repository then the main project repository.
This post by the Github team makes it sound like you should be able to link to
the Wiki is located at [repositoryname]/wiki
[repositoryname]/wiki
The files in the repository are located at [repositoryname]/raw/master/[file path in repository]
[repositoryname]/raw/master/[file path in repository]
So just use the relative path: ../raw/master/[file path in repository]
../raw/master/[file path in repository]