Replace <Link> with <a>
Created by: kevinburke
Previously clicking the links on the "help" page would redirect you to https://yourhost/https://github.com/sourcegraph/... which is incorrect. Link to github instead.
Tested locally by reproducing the error, then checking out the branch, fixing the link and verifying it went to the correct place.
Fixes sourcegraph/sourcegraph#154.
This PR does not need to update the CHANGELOG because it's a pretty small fix.