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MINOR: [Docs][C++] Fix a typo (#38625)



### Rationale for this change

Noticed a typo on the web doc and proposing a fix

### Are these changes tested?

No. But the PR is changing only text.

### Are there any user-facing changes?

Yes. But only documentation typo

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>