ARROW-8851: [Python][Documentation] Fix FutureWarnings in Python Plas…
ARROW-8851: [Python][Documentation] Fix FutureWarnings in Python Plas…

…ma docs

Signed-off-by: Weston Steimel <fc0bd5ef2e1a4c569ce03354362e47e1ec1f66bd@gmail.com>

Closes #7217 from westonsteimel/python-plasma-doc-updates

Authored-by: Weston Steimel <fc0bd5ef2e1a4c569ce03354362e47e1ec1f66bd@gmail.com>
Signed-off-by: François Saint-Jacques <76801b7d5fe2c448a8401d2246372be49d372480@gmail.com>

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