MINOR: [C++][Docs] Fixing a typo in scanner MaterializedFields function docs (#14309)
MINOR: [C++][Docs] Fixing a typo in scanner MaterializedFields function docs (#14309)

There is a typo in the one of the expressions and fixed in this PR.

Authored-by: Vibhatha Lakmal Abeykoon <3e0a2c3ede2601b97e20ad59abb206bcc32e9846@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>
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