fix relative URLs

@arowla This fixes a few URLs from the new config in https://github.com/18F/fbopen/pull/163. The site works for me at http://127.0.0.1:4000/fbopen/, though links like http://127.0.0.1:4000/fbopen/apidocs don't because the local Jekyll server doesn't handle missing file extensions like GitHub Pages does (http://18f.github.io/fbopen/apidocs). Hope that helps!