Fix broken Ethos-U tutorial reference in embedded-section

[rascani]

Summary

The tutorial was moved from docs/source/tutorial-arm-ethos-u.md to docs/source/backends/arm-ethos-u/tutorials/ethos-u-getting-started.md by docgen but this reference was not updated.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18382

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 00eb545 with merge base 7134708 ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

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[github-actions]

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[rascani]

Broken: https://docs.pytorch.org/executorch/stable/embedded-section.html#tutorials
Fixed: https://docs-preview.pytorch.org/pytorch/executorch/18382/embedded-section.html#tutorials