ARROW-8913: [Ruby] Use "field" instead of "child"
ARROW-8913: [Ruby] Use "field" instead of "child"

Closes #7255 from kou/ruby-use-field

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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