Update XNNPACK docs to use to_edge_transform_and_lower API (#5344)

Summary:
Quick doc update to use the new to_edge_transform_and_lower API, since we recommend this path now.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5344

Test Plan: Rendered doc for this PR: https://docs-preview.pytorch.org/pytorch/executorch/5344/tutorial-xnnpack-delegate-lowering.html.

Reviewed By: mcr229

Differential Revision: D62634494

Pulled By: GregoryComer

fbshipit-source-id: c28881a8be5b6398da6d506819c28d085ff2762e