ARROW-15369: [Doc] Tweak example to use the new support for str pointers
ARROW-15369: [Doc] Tweak example to use the new support for str pointers

Closes #12495 from amol-/ARROW-15369

Authored-by: Alessandro Molina <15e43f79e4563fde6c3452592b20e2241738524d@turbogears.org>
Signed-off-by: Alessandro Molina <15e43f79e4563fde6c3452592b20e2241738524d@turbogears.org>

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