ARROW-13462: [C++] Fix example code stub in Compute API documentation
ARROW-13462: [C++] Fix example code stub in Compute API documentation

Closes #11636 from 9prady9/ARROW-13462-Incorrect-examples-in-compute-rst

Authored-by: Pradeep Garigipati <7875144935e72994ad8d49602cc93890966c04a8@gmail.com>
Signed-off-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>

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