Fix generated_file walk_keys example.

It was missing an '=' and 'walk_keys'. It had an extraneous
'my_extra_files'. Add and remove these respectively.

Change-Id: Ifab92c41e3aa143264f98da177fa18f51dbaca10
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/9160
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

src/gn/metadata.cc

diff --git a/src/gn/metadata.cc b/src/gn/metadata.cc
index 236ceeb..fa52081 100644
--- a/src/gn/metadata.cc
+++ b/src/gn/metadata.cc
@@ -93,7 +93,7 @@
     group("a") {
       metadata = {
         my_files = [ "foo.cpp" ]
-        my_files_barrier [ ":b" ]
+        my_files_barrier = [ ":b" ]
       }
 
       deps = [ ":b", ":c" ]
@@ -113,7 +113,8 @@
 
     generated_file("metadata") {
       outputs = [ "$root_build_dir/my_files.json" ]
-      data_keys = [ "my_files", "my_extra_files" ]
+      data_keys = [ "my_files" ]
+      walk_keys = [ "my_files_barrier" ]
 
       deps = [ ":a" ]
     }