Fix a typo in gn help.

R=brettw@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2595503003
Cr-Original-Commit-Position: refs/heads/master@{#439851}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 889ccd0f81e2387c4de915f9e915438a7ed432c1