DOC/TST: Fix warning in concat docstring

[TomAugspurger]

xref #23149

[pep8speaks]

Hello @TomAugspurger! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/core/reshape/concat.py !

[codecov]

Codecov Report

Merging #23529 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #23529      +/-   ##
==========================================
+ Coverage   92.25%   92.25%   +<.01%     
==========================================
  Files         161      161              
  Lines       51181    51186       +5     
==========================================
+ Hits        47217    47222       +5     
  Misses       3964     3964

Flag	Coverage Δ
#multiple	90.64% <ø> (ø)	⬆️
#single	42.27% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/reshape/concat.py	97.6% <ø> (ø)	⬆️
pandas/core/groupby/groupby.py	96.51% <0%> (+0.02%)	⬆️

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[datapythonista]

lgtm

checked the log, and seems like it fixes the specifying 'categories' or 'ordered' in .astype() is deprecated; pass a CategoricalDtype instead warning