Migrations Guide: Fix line length [ci skip]

Inline code in Chapter 7.2 is extending to 2 lines.

![migrations-guide-screen-shot-2013-09-28-at-3 53 16-am](https://f.cloud.github.com/assets/360276/1230751/fb4b0b3a-2802-11e3-991d-9cf073f7dc44.png)