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
```