Replace associationMappings to fieldMappings for association property at Guesser

Without this fix for property:
```php
/**
* @MongoDB\ReferenceOne(targetDocument="ArticleCategory", nullable=true)
*/
protected $parent;
```
I have error notice:
```html
Notice: Undefined index: parent in /var/www/exercise/vendor/sonata-project/doctrine-mongodb-admin-bundle/Sonata/DoctrineMongoDBAdminBundle/Guesser/TypeGuesser.php on line 34

Notice: Undefined index: parent in /var/www/exercise/vendor/sonata-project/doctrine-mongodb-admin-bundle/Sonata/DoctrineMongoDBAdminBundle/Guesser/FilterTypeGuesser.php on line 43
```