MINOR: [C++] Fix IndexIn example in docstring
MINOR: [C++] Fix IndexIn example in docstring

https://github.com/apache/arrow/pull/10174 (ARROW-12554) updated the functionality to properly map the column index when repeated values are present.  This updates the docs to match.

Closes #11422 from emkornfield/fix-IndexIn-docs

Authored-by: emkornfield <dbc64ab8a675880673029ed6b2cb1d084dfef0bb@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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