Correct quoting/code block of documentation for 'salt.states.reg.present()'

Currently, [this renders](http://docs.saltstack.com/en/latest/ref/states/all/salt.states.reg.html) a bit wrong (no backslashes, code rendered a RST instead of plain YAML etc.).

This PR should fix this.

Thanks @tedski for suggestions on how to handle backslashes correctly.