webrick/utils.rb: mark by class name

* lib/webrick/utils.rb (WEBrick::Utils::TimeoutHandler::Thread):
  use particular class to mark by the class name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46651 b2dd03c8-39d4-4d8f-98ff-823fe69b080e