Fixes downloadableItems accessibility bug.

[kidunot89]

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What does this implement/fix? Explain your changes.

• changes "downloadableItem" to "downloadable_item" is the allowed_restricted_fields list in the Order model to make the model field name.

Does this close any currently open issues?

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Any relevant logs, error output, GraphiQL screenshots, etc?

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Any other comments?

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Where has this been tested?

Operating System: Ubuntu 20.04

WordPress Version: 5.4.2