Changed print to plot in gaussian_threshold.md

[JoeDuenwald]

Describe your changes
Changed pcv.params.debug = "print" to pcv.params.debug = "plot"

Type of update

• Update to documentation

Associated issues
Issue #999

Additional context
N/A

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

[codecov]

Codecov Report

Merging #1095 (88df8b3) into main (860554d) will not change coverage.
The diff coverage is n/a.

❗ Current head 88df8b3 differs from pull request most recent head 25deb91. Consider uploading reports for the commit 25deb91 to get more accurate results

@@            Coverage Diff            @@
##              main     #1095   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          141       141           
  Lines         6003      6003           
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  Hits          6003      6003           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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