Clean up link references in //tools/gn/README.md.

Before Gitiles bug 82 was fixed, Gitiles did not render relative URLs
correctly on branches, so the GN docs had hard-coded absolute URLs.
That bug is now fixed, so this updates the docs.

R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2483673002
Cr-Original-Commit-Position: refs/heads/master@{#439388}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 44dbd2f12ae5d6307b607f18fe2dd466d8a8fa38
1 file changed
tree: 182e332f1d7c07b17c77e89d15f92f221493ac8e
  1. tools/