[GN] Fix gn gen docs.

Use the same "gn $cmd $args" format for the first line in the long docs
for gn gen as the other gn cmds.. The previous doc resulted in an incorrect
anchor of 'gen:' getting generated.

BUG=752189

Change-Id: I2366b64794a524c77d38a23776cf3860eee07ee0
Reviewed-on: https://chromium-review.googlesource.com/600538
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Mike Bjorge <mbjorge@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#491892}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 679d858fb7e37623c04ffcd3a0b66d05818868dd
2 files changed
tree: 7f90a701aa470dec0b0783b88b3982b02f8dd918
  1. tools/