Fix missing closing brackets [ci skip]

I'm so happy about new version. Currently our http://www.sinatrarb.com/intro-ja.html looks horrible.
I've fixed this and it is fine now. Could you update at https://github.com/sinatra/sinatra.github.com or send another pull request?


![before](http://s27.postimg.org/6eslpo9zn/Screen_Shot_2014_04_09_at_8_57_42_AM.png)
![after](http://s28.postimg.org/hykz4xa8t/Screen_Shot_2014_04_09_at_9_03_43_AM.png)