[gn] Fix typos in the documentation. BUG= Review URL: https://codereview.chromium.org/1579133002 Cr-Original-Commit-Position: refs/heads/master@{#369420} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: b541f71072bc92cfcf37ed9b136c6af2c85cfdf9