Fix import path for DataSubject in L2 notebook

[akalliokoski]

Description

This change fixes DataSubject import path in L2 notebook.

Affected Dependencies

N/A

How has this been tested?

• Run the edited code block in Jupyter Lab on my local machine

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests

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