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MINOR: [C++] Fix the simple typo in the test (#44747)



### Rationale for this change

There is a minor typo in the test case.

### What changes are included in this PR?

Fix the simple typo in the test.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Authored-by: c8ef <c8ef@outlook.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>