Fix typo in gn documentation

Also updates an improper usage in a BUILD.gn file.

R=dpranke
BUG=None

Change-Id: I2d7d96f29165442e8642991a6db942b17b6c5012
Reviewed-on: https://chromium-review.googlesource.com/1069745
Reviewed-by: David Turner <digit@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#561031}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 524e60fb52e11d589f3a0cc9010e98080bf98e23