Missing reference in code example

[SimonDesbonnets]

Adding a missing reference to avoid getting a compilation error

Proposed change(s)

Describe the changes made in this PR.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• [X] Bug fix
• [ ] New feature
• [ ] Code refactor
• [ ] Breaking change
• [ ] Documentation update
• [ ] Other (please describe)

Checklist

• [ ] Added tests that prove my fix is effective or that my feature works
• [ ] Updated the changelog (if applicable)
• [ ] Updated the documentation (if applicable)
• [ ] Updated the migration guide (if applicable)

Other comments

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