changed print to plot in apply_mask

[lacostag]

Describe your changes
changed print to plot

Type of update

• 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

[JoeDuenwald]

Looks good!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@a061598). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head d7977c3 differs from pull request most recent head fb84e78. Consider uploading reports for the commit fb84e78 to get more accurate results

@@           Coverage Diff           @@
##             main    #1027   +/-   ##
=======================================
  Coverage        ?   99.98%           
=======================================
  Files           ?      141           
  Lines           ?     6004           
  Branches        ?        0           
=======================================
  Hits            ?     6003           
  Misses          ?        1           
  Partials        ?        0           

Flag	Coverage Δ
unittests	99.98% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.