fix typo

searched with below code(c.f. まつもとゆきひろ コードの世界)

  #!/usr/bin/ruby

  ARGF.each do |line|
    print ARGF.file.path, " ",
      ARGF.file.lineno, ":",
      line if line.gsub!(/([へにおはがでな])\1/s, '[[\&]]')
  end