fix(sample): Fix missing --region in dataflow sample notebook. Fixes #5007

[chensun]

Description of your changes:
Following #4960, Dataflow component now requires a --region argument.

Fixes post-submit test: #5007

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[chensun]

/retest

[Bobgy]

Awesome! Thank you for the fix!
/lgtm
/approve

[k8s-ci-robot]

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[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Bobgy, chensun

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• samples/OWNERS [Bobgy]
• test/sample-test/OWNERS [Bobgy]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment