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MINOR: [Docs][Java] Fix incorrect example (#39941)

### Rationale for this change

`setRowCount` should be called after filling vectors.

### What changes are included in this PR?

Move `setRowCount` to after filling vectors.

### Are these changes tested?

No

### Are there any user-facing changes?

Documentation

Authored-by: wforget <643348094@qq.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>