doc: Fix typo in example

[ppflrs]

Fix typo in the example for pandas.io.formats.style.Styler.format

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19537 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19537      +/-   ##
==========================================
- Coverage   91.62%    91.6%   -0.03%     
==========================================
  Files         150      150              
  Lines       48717    48717              
==========================================
- Hits        44637    44625      -12     
- Misses       4080     4092      +12

Flag	Coverage Δ
#multiple	89.97% <ø> (-0.03%)	⬇️
#single	41.75% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/formats/style.py	96.21% <ø> (ø)	⬆️
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️

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[TomAugspurger]

Thanks @ppflrs!