changed print to plot in spectral_index

[sdkenney42]

Describe your changes
updated print to plot in spectral_index

Type of update
Is this a:

• Update to documentation

Associated issues
#999

Additional context
Add any other context about the problem here.

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 #1059 (a89abc4) into main (f84cd8f) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #1059   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          141       141           
  Lines         6002      6003    +1     
=========================================
+ Hits          6002      6003    +1     

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

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see 1 file with indirect coverage changes