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GH-43870: [C++][Acero] Fix typos in join benchmark (#43871)



### Rationale for this change

These are rather obvious typos.

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

* GitHub Issue: #43870

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>