Typo fix to mask in thermal_hist

[lchavez037]

Describe your changes
Typo fix to mask in thermal_hist.

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

[JoeDuenwald]

All looks good.

[codecov]

Codecov Report

Merging #1011 (bac2e47) into main (ecb4877) will not change coverage.
The diff coverage is n/a.

❗ Current head bac2e47 differs from pull request most recent head a05c05e. Consider uploading reports for the commit a05c05e to get more accurate results

@@           Coverage Diff           @@
##             main    #1011   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         141      141           
  Lines        6004     6004           
=======================================
  Hits         6003     6003           
  Misses          1        1           

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

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[JoeDuenwald]

Approved