Fixed a code block.

Last code block in the [Keeping your Modules independent](http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/cookbook/resolve-target-entity-listener.html) cookbook is broken.

Sphinx does not like the way code was indented. Building the documentation raises the following error:

    en/cookbook/resolve-target-entity-listener.rst:121: ERROR: Unexpected indentation.