Fix prune command in docker buildx du

[mbergen]

Description

The command to prune data is docker buildx prune, not docker buildx du prune (which fails with ERROR: "docker buildx du" accepts no arguments.)

Reviews

• Technical review
• Editorial review
• Product review

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[docker-robot]

Thanks for the pull request. We'd like to make our product docs better, but haven’t been able to review all the suggestions.
As our docs have also diverged, we do not have the bandwidth to review and rebase old pull requests.

If the updates are still relevant, review our contribution guidelines and rebase your pull request against the latest version of the docs, then mark it as fresh with a /remove-lifecycle stale comment.
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