Fix: ManyToManyRelation have wrong phpdocs when use print documents

[blankse]

The \ is missing at the begin of the class name.

Before:

/**
* Get sources - testclass.testfield
* @return Pimcore\Bundle\WebToPrintBundle\Model\Document\Printpage[]|Pimcore\Bundle\WebToPrintBundle\Model\Document\Printcontainer[]
*/
public function getTestfield(): array
{
    ...
}

After:

/**
* Get sources - testclass.testfield
* @return \Pimcore\Bundle\WebToPrintBundle\Model\Document\Printpage[]|\Pimcore\Bundle\WebToPrintBundle\Model\Document\Printcontainer[]
*/
public function getTestfield(): array
{
    ...
}