ARROW-15143: [C++] Remove incorrect comment on API of Transform for StringBinaryTransformExecBase
ARROW-15143: [C++] Remove incorrect comment on API of Transform for StringBinaryTransformExecBase

Remove incorrect comment on the signature of `Transform` in `StringBinaryTransformExecBase`-derived classes.

Closes #12029 from edponce/ARROW-15143-Remove-incorrect-comment-on-API-of-Transform-StringBinary

Authored-by: Eduardo Ponce <6a4c8f271abaad6949809c569bf8d3844179d012@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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