Update xnnpack_README.md

[cmodi-meta]

Correcting command in readme for exporting model.

[pytorch-bot]

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[kirklandsign]

@pytorchbot label "topic: not user facing"

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[github-actions]

Looks like this PR hasn't been updated in a while so we're going to go ahead and mark this as Stale.
Feel free to remove the Stale label if you feel this was a mistake.
If you are unable to remove the Stale label please contact a maintainer in order to do so.
If you want the bot to never mark this PR stale again, add the no-stale label.
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