Change args.gn to reference `gn help buildargs`

Executing `gn buildargs` is an error:

  $ gn buildargs
  ERROR Command "buildargs" unknown.

Change-Id: I53d9d50b1ca967bb26e673d1317246a73f2fb85d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6520
Reviewed-by: Haowei Wu <haowei@google.com>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

src/gn/setup.cc

diff --git a/src/gn/setup.cc b/src/gn/setup.cc
index f0796d5..4390f0a 100644
--- a/src/gn/setup.cc
+++ b/src/gn/setup.cc
@@ -150,7 +150,7 @@
 namespace {
 
 const base::FilePath::CharType kGnFile[] = FILE_PATH_LITERAL(".gn");
-const char kDefaultArgsGn[] = "# Set build arguments here. See `gn buildargs`.";
+const char kDefaultArgsGn[] = "# Set build arguments here. See `gn help buildargs`.";
 
 base::FilePath FindDotFile(const base::FilePath& current_dir) {
   base::FilePath try_this_file = current_dir.Append(kGnFile);