Correct gn arg in instructions

The webui_closure_compile instructions refer to closure_compile but the
correct gn arg name (as of crrev.com/c/2248564) is enable_js_type_check.

Change-Id: I7bd947e0a277fe6e0f4972cd69ac2fc1c0b9b8a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335557
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794582}

docs/closure_compilation.md

@@ -120,7 +120,8 @@ You can locally test that your code compiles on Linux or Mac.  This requires
 python, depot_tools). Note: on Ubuntu, you can probably just run `sudo apt-get
 install openjdk-7-jre`.
 
-After you set closure_compile = true in your gn args, you should be able to run:
+After you set enable_js_type_check = true in your gn args, you should be able to
+run:
 
 ```shell
 ninja -C out/Default webui_closure_compile