[cookbook][email] fixed template file (added .txt.twig)

Author: hidenorigoto

cookbook/email.rst

@@ -109,7 +109,7 @@ an email is pretty straightforward::
  ->setSubject('Hello Email')
  ->setFrom('send@example.com')
  ->setTo('recipient@example.com')
- ->setBody($this->renderView('HelloBundle:Hello:email', array('name' => $name)))
+ ->setBody($this->renderView('HelloBundle:Hello:email.txt.twig', array('name' => $name)))
  ;
  $mailer->send($message);