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MINOR: [C++] Fix a typo (#39053)



### Rationale for this change

Noticed a typo on the code and proposing a fix

### Are these changes tested?

Yes. It is passed by existing test cases.

### Are there any user-facing changes?

No

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>