Guide for Action Mailer Basics: Un-inline if statement

**on [rails/rails/master](https://github.com/rails/rails/blob/d233220f2921472c85142137d63f42d2d728adaa/guides/source/action_mailer_basics.md#intercepting-emails)**

The single line is long enough that it renders on two lines, causing the example to look like syntactically invalid code.

![screenshot 2014-12-29 13 07 35](https://cloud.githubusercontent.com/assets/77495/5572439/ae770306-8f5b-11e4-9a4f-f084aedf0272.png)

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**on [JoshCheek/rails/mailer_guide_code_example](https://github.com/JoshCheek/rails/blob/mailer_guide_code_example/guides/source/action_mailer_basics.md#intercepting-emails)**

Switch to multi-line if statement so that the code sample doesn't look invalid

![screenshot 2014-12-29 13 06 56](https://cloud.githubusercontent.com/assets/77495/5572435/9962b78a-8f5b-11e4-8d36-2db42c292f44.png)