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ARROW-7459: [Python] Fix document lint error

This is included by e519853fbfd0241d87572d8b333a8ab34f02b401.

I missed this. Sorry.

Closes #6085 from kou/python-parquet-fix-lint and squashes the following commits:

0d5b5263c <Sutou Kouhei>  Fix document lint error

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>