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GH-38977: [C++] Fix spelling (#38978)



### Rationale for this change

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #38977

Authored-by: Josh Soref <2119212+jsoref@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>