MINOR: [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 API page at arrow.apache.org/docs/cpp/api/formats.html shows false.
• Default was changed in a previous pr but comment/doc was not updated

What changes are included in this PR?

• Updated the default value

Are these changes tested?

• N/A - Docs

Are there any user-facing changes?

• Yes

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[ArnavBalyan]

cc @pitrou thanks! :)

[pitrou]

Also opened #49923 for the corresponding Python options.

[pitrou]

+1, thank you @ArnavBalyan

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 0 benchmarking runs that have been run so far on merge-commit a2d8e9c.

None of the specified runs were found on the Conbench server.

The full Conbench report has more details.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit a2d8e9c.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 1 possible false positive for unstable benchmarks that are known to sometimes produce them.