Fix grammar-o in gn's help.

Change-Id: Id6d327ce5631c6078a45760eee7597a3dfc90416
Reviewed-on: https://chromium-review.googlesource.com/1000023
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#548882}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 743a7a884c8d1008067f662c003c7e1d31c39d73