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MINOR: [Ruby][Gandiva][Docs] Fix syntax error in example (#39551)

### Rationale for this change

Example code has syntax error.

### What changes are included in this PR?

Fix the syntax error.

### Are these changes tested?

No.

### Are there any user-facing changes?

Yes.

Authored-by: dsisnero <dsisnero@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>