MINOR: [C++][Parquet] Fix incorrect default in ArrowReaderProperties::set_pre_buffer doc #6505

Job	Run time
check-labels / Check labels	28s
ODBC Windows	0s
JNI macOS	0s
ARM64 Windows 11 MSVC	0s
ODBC release	0s
ODBC Linux	0s
ODBC ${{ matrix.build-type }} ${{ matrix.architecture }} macOS ${{ matrix.macos-version }}	0s
ODBC nightly	0s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	0s
matrix.title	0s
report-extra-cpp	0s
	28s

Provide feedback