Fix a typo in GN documentation

This CL fixes the typo introduced in r439541.

Review-Url: https://codereview.chromium.org/2591863002
Cr-Original-Commit-Position: refs/heads/master@{#439891}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a8936b7a13ff687bb186db70681723bf8b6bd531