arrow

MINOR: [Docs] Correct ListView example doc in Columnar.rst (#45265)

### Rationale for this change
The pr aims to correct an error in doc `Columnar.rst`

### What changes are included in this PR?
Obviously, the value of `Length` below is incorrect, it should be `5`.
<img width="775" alt="image" src="https://github.com/user-attachments/assets/9e53d415-0a6e-4d2b-9822-9eb20456898a" />

### Are these changes tested?
No, only for docs.

### Are there any user-facing changes?
Reduce misunderstandings among end users.

Authored-by: panbingkun <panbingkun@baidu.com>
Signed-off-by: mwish <maplewish117@gmail.com>