[Railsguides] Fix style for rescue responses doc

This is a follow-up commit to https://github.com/rails/rails/pull/17076

 * Changed *Any ***s that are...* to *Any *** that is...*
 * Fixed style for default rescue_responses setting

Although, this is not still very good:
![screen shot 2014-10-02 at 9 58 11 pm](https://cloud.githubusercontent.com/assets/386234/4501027/eafa3410-4aa0-11e4-87fa-15dd7aa449fe.png)

cc @zzak @vipulnsward