gn docs: Update toolchain example in the language reference

tools/gn/docs/language.md

 # GN Language and Operation
 
 [TOC]
 
 ## Introduction
 
 This page describes many of the language details and behaviors.
 
 ### Use the built-in help!
 
@@ skipping 589 matching lines @@
 two toolchains:
 
 ```
 toolchain("32") {
   tool("cc") {
     ...
   }
   ... more tools ...
 
   # Arguments to the build when re-invoking as a secondary toolchain.
-  toolchain_args() {
-    toolchain_cpu = "x86"
+  toolchain_args = {
+    current_cpu = "x86"
   }
 }
 
 toolchain("64") {
   tool("cc") {
     ...
   }
   ... more tools ...
 
   # Arguments to the build when re-invoking as a secondary toolchain.
-  toolchain_args() {
-    toolchain_cpu = "x64"
+  toolchain_args = {
+    current_cpu = "x64"
   }
 }
 ```
 
 The toolchain args specifies the CPU architecture explicitly, so if a
 target depends on something using that toolchain, that cpu architecture
 will be set when re-invoking the build. These args are ignored for the
 default toolchain since by the time they're known the build config has
 already been run. In general, the toolchain args and the conditions used
 to set the default toolchain should agree.
@@ skipping 170 matching lines @@
 GN keeps some GYP concept like "all dependent" settings which work a bit
 differently in Blaze. This is partially to make conversion from the existing
 GYP code easier, and the GYP constructs generally offer more fine-grained
 control (which is either good or bad, depending on the situation).
 
 GN also uses GYP names like "sources" instead of "srcs" since
 abbreviating this seems needlessly obscure, although it uses Blaze's
 "deps" since "dependencies" is so hard to type. Chromium also compiles
 multiple languages in one target so specifying the language type on the
 target name prefix was dropped (e.g. from `cc_library`).