Updated the W2 model IDs to be consistent with the current production model IDs

[rklawrence]

Updated the prebuilt W2 model ID from prebuilt-tax.us.W-2 to prebuilt-tax.us.w2.

This is consistent with the (github example)[https://github.com/Azure-Samples/document-intelligence-code-samples/tree/main/Python(v4.0)/Prebuilt_model].

This new model ID is also what is expected by Document Intelligence, while the previous model ID causes an error due to a "missing model".

[prmerger-automator]

@rklawrence : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit cc330f3:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/ai-services/document-intelligence/prebuilt/tax-document.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[Court72]

@laujan

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team