Fix return type in phpdoc

[DanieliMi]

Description (*)

The method does return a \Magento\Sales\Api\Data\OrderItemInterface and it should be typed as such. \Magento\Sales\Model\Order::getItems is called to get the item and in its phpdoc the return type is \Magento\Sales\Api\Data\OrderItemInterface[], so it only makes sense to return \Magento\Sales\Api\Data\OrderItemInterface it in these methods

Fixed Issues (if relevant)

No Issue created.

Manual testing scenarios (*)

No manual testing steps necessary this should be logical in the review.

Questions or comments

Contribution checklist (*)

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• All automated tests passed successfully (all builds are green)

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