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MINOR: [C++][Benchmark][Acero] Fix a typo (#34875)

### Rationale for this change

It causes a build error:

```text
cpp/src/arrow/acero/hash_join_benchmark.cc:40:3: error: ‘JoinType’ does not name a type; did you mean ‘UnionType’?
   40 |   JoinType join_type = JoinType::INNER;
      |   ^~~~~~~~
      |   UnionType
...
```

### What changes are included in this PR?

Fix a typo.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.