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GH-41192: [C++] Fix hashjoin benchmark failed at make utf8's random batches (#41195)



### Rationale for this change
Fix hashjoin benchmark failed at make utf8's random batches.

### What changes are included in this PR?
Add to_string for `min_length` and `max_length`'s value.

### Are these changes tested?
Needn't

### Are there any user-facing changes?
No

* GitHub Issue: #41192

Authored-by: ZhangHuiGui <hugo.zhang@openpie.com>
Signed-off-by: mwish <maplewish117@gmail.com>