DoctrineORMFieldGuesser - ManyToMany relations

When using the `avocode/form-extensions-bundle` to use a double_list on a ManyToMany Doctrine relation, the generated formOptions was not correct : the `multiple` formOption was set to `false` and the entity could not be saved correctly.

I added a test on the relation type in the DoctrineORMFieldGuesser to set `multiple` to `true` in the case of a ManyToMany relation.

My tests were made with the following config:
```
admingenerator_generator:
    form_types:
        doctrine_orm:
            collection:   double_list_entity
```

Do you think this fix is correct ?