Added namespace declaration to MainController.php.

Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | 

My first experience with Symfony so I'm going through the docs and entering the code snippets. I got an exception when trying to view the newly-created contact page in HTTP Fundamentals - saying that it found the MainController file but couldn't find the class. I added a missing namespace declaration and that fixed the problem.