gn docs: Update toolchain example in the language reference

Since https://codereview.chromium.org/2219083002 the syntax for
declaring toolchain arguments has changed, so update the
example:
- Use toolchain_args = {} instead of toolchain_args() {}
- Use current_cpu instead of toolchain_cpu.

R=brettw@chromium.org,dpranke@chromium.org
BUG=634446

Review-Url: https://chromiumcodereview.appspot.com/2430153004
Cr-Original-Commit-Position: refs/heads/master@{#426228}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fe6025a7bcbba301acfe8a049c44b17f0b19efd5