Fix typo in error message in the PlanePlot class

[CianciuStyles]

• closes Typo in error message in the PlanePlot class #21347
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Very simple fix, just a typo in an error message.

[codecov]

Codecov Report

Merging #21350 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #21350   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45512    45512           
  Misses       4037     4037

Flag	Coverage Δ
#multiple	90.25% <0%> (ø)	⬆️
#single	41.87% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_core.py	82.39% <0%> (ø)	⬆️

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[jreback]

thanks!