Minor example code fix

[mlohbihler]

Describe your changes
The example code for updating to version 4 appears to have a small bug. The variable bin_img is not defined before it is used, and the variable bin_mask is unused after defining. Moreover, making the change allows the sample code to run completely.

Type of update
Is this a sample code bug fix.

Associated issues
None

Additional context
None

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 1dbebaf..b4319fb. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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[HaleySchuhl]

Thank you @mlohbihler for catching and fixing this!

[HaleySchuhl]

@all-contributors please add @mlohbihler for bug and documentation

[allcontributors]

@HaleySchuhl

I've put up a pull request to add @mlohbihler! 🎉

[mlohbihler]

Thank you @mlohbihler for catching and fixing this!

@HaleySchuhl No trouble at all! There seems to have been a timeout in the test coverage though, so i can't merge. Can you do so?