Update ubuntu.md(Added missed Step)

[bigpreshy]

A step was missed. I just added the step

Description

After you download the .deb file for Docker desktop, you need to move to the directory where the file is saved before moving to the next step.

Related issues or tickets

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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.
If not, this pull request will be closed in 30 days. This helps our maintainers focus on the active pull requests.

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/lifecycle stale