Sample values for String data structures include newlines

[abtris]

@kylef opened apiaryio/snowcrash#378

Given the following example:

## Data Structures

### datetime (string)

An ISO8601 formatted date.

#### Sample

2015-10-15T13:21:09Z

Where we have a sample value of 2015-10-15T13:21:09Z, we include two \n (new lines) in the rendered JSON.