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GH-38883: [Docs] Fix struct example to show hiding a child's entry (#38898)



### Rationale for this change
See the issue.

### What changes are included in this PR?
The struct example now demonstrates what it's supposed to.

### Are these changes tested?
Renders well

### Are there any user-facing changes?
Just docs.

* Closes: #38883

Authored-by: Stas Stepanov <78556261+stfdxv@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>