Improve readability of Explain Queries table in guides

In the ActiveRecord Querying section of the guides, there is a Running Explain section which shows the output from running explain on an AR query. The table output in the guide currently looks quite jumbled and is quite hard to understand.

## Here's how it looks currently:
![active_record_query_interface__ruby_on_rails_guides](https://cloud.githubusercontent.com/assets/2823694/3075760/b35cb5bc-e391-11e3-9d60-0921728346e1.png)
![active_record_query_interface__ruby_on_rails_guides](https://cloud.githubusercontent.com/assets/2823694/3075761/b91b2e70-e391-11e3-90f0-d1bc24d6f575.png)

## Here's how it looks with this PR's changes
![active_record_query_interface__ruby_on_rails_guides](https://cloud.githubusercontent.com/assets/2823694/3075762/d3b1f93a-e391-11e3-9958-6915bd70b10a.png)
![active_record_query_interface__ruby_on_rails_guides](https://cloud.githubusercontent.com/assets/2823694/3075763/d807f660-e391-11e3-934a-163670284eac.png)