Change print to plot in skeletonize.md

[lchavez037]

Describe your changes
Change print to plot in skeletonize.

Type of update
Is this a:

• Update to documentation

Associated issues
issue #999

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

[codecov]

Codecov Report

Merging #1043 (2563513) into main (ea27ec0) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 2563513 differs from pull request most recent head 64085e9. Consider uploading reports for the commit 64085e9 to get more accurate results

@@             Coverage Diff             @@
##              main    #1043      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files          141      141              
  Lines         6003     6004       +1     
===========================================
  Hits          6003     6003              
- Misses           0        1       +1     

Flag	Coverage Δ
unittests	99.98% <ø> (-0.02%)	⬇️

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see 1 file with indirect coverage changes