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

tools/gn/docs/language.md

diff --git a/tools/gn/docs/language.md b/tools/gn/docs/language.md
index e6e80d3..514541d 100644
--- a/tools/gn/docs/language.md
+++ b/tools/gn/docs/language.md
@@ -607,8 +607,8 @@
   ... more tools ...
 
   # Arguments to the build when re-invoking as a secondary toolchain.
-  toolchain_args() {
-    toolchain_cpu = "x86"
+  toolchain_args = {
+    current_cpu = "x86"
   }
 }
 
@@ -619,8 +619,8 @@
   ... more tools ...
 
   # Arguments to the build when re-invoking as a secondary toolchain.
-  toolchain_args() {
-    toolchain_cpu = "x64"
+  toolchain_args = {
+    current_cpu = "x64"
   }
 }
 ```