Fix the header link on plugins

Summary: The link used to be relative, leading to links like http://vimawesome.com/plugin/plugin/undotree-vim which weren't valid. This makes the links absolute, so they should work better. Fixes #16.

Test Plan: Hope to god that this works

Reviewers: divad12, spicyj

Reviewed By: spicyj

Subscribers: divad12

Differential Revision: http://phabricator.benalpert.com/D204