Update object_detector.ipynb: fix a typo

[martin0258]

Description

Just fix a typo in the Colab notebook.

Checklist

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• My code follows the code style of the project.
• I have updated the documentation (if applicable).
• I have added tests to cover my changes.

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• Bug fix (non-breaking change which fixes an issue)
• Documentation update (non-breaking change which updates documentation)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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