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GH-39336: [C++][Parquet] Minor: Style enhancement for parquet::FileMetaData (#39337)

### Rationale for this change

Enhance the style of `parquet::FileMetaData::Subset`.

### Are these changes tested?

Already tested

### Are there any user-facing changes?

no

* Closes: #39336

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: mwish <1506118561@qq.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: mwish <maplewish117@gmail.com>