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ARROW-10749: [C++] Incorrect string format for Datum with the collection type

The current format looks like this: `Collection(...`, the right embrace is omitted.

Closes #8783 from liyafan82/fly_1127_str

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>