MINOR: [Docs] Add an empty line to make .. code-block:: work correc…

…tly (apache#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>

docs/source/developers/java/development.rst

@@ -42,6 +42,7 @@ Unit Testing
 Unit tests are run by Maven during the build.
 
 To speed up the build, you can skip them by passing -DskipTests.
+
 .. code-block::
 
     $ cd arrow/java