DOC: minor documentation fix on frame.dropna

[minggli]

• closes DOC: minor deprecate message documentation on frame.dropna #21209
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff

After this PR, it should look like this:

[codecov]

Codecov Report

Merging #21210 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21210   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039

Flag	Coverage Δ
#multiple	90.23% <ø> (ø)	⬆️
#single	41.88% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.22% <ø> (ø)	⬆️

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[TomAugspurger]

Thanks!