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e7471f85 - MINOR: [C++] Fix typos in key_map doc and comment (#45104)

MINOR: [C++] Fix typos in key_map doc and comment (#45104) ### Rationale for this change ### What changes are included in this PR? ### Are these changes tested? ### Are there any user-facing changes? Authored-by: Rossi Sun <zanmato1984@gmail.com> Signed-off-by: Rossi Sun <zanmato1984@gmail.com>