MINOR: [Docs] Add an empty line to make `.. code-block::` work correctly (#39388)

### Rationale for this change

Code block [here](https://arrow.apache.org/docs/developers/java/development.html#unit-testing) didn't work correctly. Added a empty line to make it work well.

### What changes are included in this PR?

Added a empty line to make it work correctly.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: John <thespica@qq.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>