[C++][Parquet] Fix incorrect default in ArrowReaderProperties::set_pre_buffer doc

[ArnavBalyan]

Rationale for this change

• The docs comment on ArrowReaderProperties::set_pre_buffer says default false, but the constructor initializes pre_buffer_(true). The published API page at arrow.apache.org/docs/cpp/api/formats.html contradicts the actual default.
• Default was changed in a previous pr but comment/doc was not updated

What changes are included in this PR?

• Update default

Are these changes tested?

• Yes

Are there any user-facing changes?

• Yes

[github-actions]

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[ArnavBalyan]

cc @pitrou thanks! :)