CLN GH23204 remove "not" typo in assertion

[rdmontgomery]

• closes Misleading "not" typo in assertion in pandas.concat in join_axes check #23204
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Remove an unwanted "not" in the assertion message.

This is my first pandas PR! Let me know if there's anything else I need to do. Thanks!

[pep8speaks]

Hello @rdmontgomery! Thanks for submitting the PR.

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

[codecov]

Codecov Report

Merging #23221 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #23221   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50962    50962           
=======================================
  Hits        46984    46984           
  Misses       3978     3978

Flag	Coverage Δ
#multiple	90.61% <ø> (ø)	⬆️
#single	42.26% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/reshape/concat.py	97.6% <ø> (ø)	⬆️

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[jreback]

thanks!