Link to the github project

The link "Fork Me on Github" was linking to github.com instead of https://github.com/a11yproject/a11yproject.com

Rather than setting the _config for `site.github.username`, just modify the template.  Setting the username would not link to the specific project, but rather https://github.com/a11yproject/

Another option is to set `site.github.username` to `a11yproject/a11yproject.com`, which seems weird because that isn't a "username".