Changed print to plot in extract_wavelength.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 #1094 (a1c9edf) into main (dcdef2f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #1094   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          141       141           
  Lines         6003      6003           
=========================================
  Hits          6003      6003           

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

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