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MINOR: [C++] Fix typo for IWYU pragma (#36140)

### Rationale for this change

The API file contains "IYWU" rather than "IWYU" for the export pragma.

### What changes are included in this PR?

The typos are corrected.

### Are these changes tested?

Since these are only comment changes, I did not test the change.

### Are there any user-facing changes?

No.

Authored-by: Mats Kindahl <mats@timescale.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>