[gn] Doc: Fix toochain typo and clarify the toolchain syntax.

Change-Id: Ica4288a6984f296958b7f9b812edb07663cec22e
Reviewed-on: https://chromium-review.googlesource.com/1104159
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#568046}
2 files changed