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GH-44677: [C++][Acero] Enhance partition sort example (#44678)

### Rationale for this change

partition sort example:
out_arr value is row id not actual value.

### What changes are included in this PR?

- partition_util.h

### Are these changes tested?

no

### Are there any user-facing changes?

comment change
* GitHub Issue: #44677

Authored-by: light-city <455954986@qq.com>
Signed-off-by: mwish <maplewish117@gmail.com>