add ML ref.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18013 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_thread.rb

@@ -360,4 +360,4 @@ assert_equal 'ok', %q{
   t = Thread.new {`echo`}
   t.join
   $? ? :ng : :ok
-}
+}, '[ruby-dev:35414]'