Fix invalid PHPDocs return comment

[michalbundyra]

As per Psalm error:

ERROR: InvalidDocblock - vendor/willdurand/negotiation/src/Negotiation/AbstractNegotiator.php:60:5 - Unexpected token [ in docblock for Negotiation\AbstractNegotiator::getOrderedElements (see https://psalm.dev/008)
    /**
     * @param string $header A string containing an `Accept|Accept-*` header.
     *
     * @return [AcceptHeader] An ordered list of accept header elements
     */
    public function getOrderedElements($header)

[willdurand]

thanks