Add missing cfg="host" to example executable



Closes #2129.

--
Reviewed-on: https://github.com/bazelbuild/bazel/pull/2129
MOS_MIGRATED_REVID=140134637
diff --git a/site/versions/master/docs/skylark/cookbook.md b/site/versions/master/docs/skylark/cookbook.md
index a6af1bf..ce3aa62 100644
--- a/site/versions/master/docs/skylark/cookbook.md
+++ b/site/versions/master/docs/skylark/cookbook.md
@@ -367,7 +367,7 @@
   attrs={
       "srcs": attr.label_list(allow_files=True),
       "out": attr.output(mandatory=True),
-      "_merge_tool": attr.label(executable=True, allow_files=True,
+      "_merge_tool": attr.label(executable=True, cfg="host", allow_files=True,
                                 default=Label("//pkg:merge"))
   }
 )