Update extending-the-model-blog-comments.rst

fixing similar problem reported here:

```
Unexpected token "name" of value "with" ("end of statement block" expected) in BloggerBlogBundle:Blog:show.html.twig at line 25
```

https://github.com/sonata-project/SonataDoctrineORMAdminBundle/issues/166

it being related to this change: https://github.com/doctrine/DoctrineBundle/issues/141