Update domain mapping chanpter for use without plugins

Hi Tom.
A small update for the Domain Mapping chapter. This works on default of WP Core, no plugins necessary. Only for Domain Mapping is a plugin necessary.

Example:
![domain-mapping](https://cloud.githubusercontent.com/assets/133425/4704453/3a9dc4bc-5872-11e4-9c63-d102d5d0dd2c.png)


Best