Fix edit path for files in folders

The edit link currently breaks for files in folders. If you visit https://project-open-data.cio.gov/v1.1/schema/ and click "Help Improve this Content", it will take you to the editing screen for the top-level `schema.md`, not `v1.1/schema.md`.

This uses `page.path` instead of `page.filename`, which incorporates the full path to that file. It's [documented here](http://jekyllrb.com/docs/variables/#page-variables) and cites formulating edit links as a main use case.