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MINOR: [Docs] Fix wrong list formatting for Integration Testing documentation (#43090)



### Rationale for this change

Fix wrong list formatting in specification for Integration Testing

### What changes are included in this PR?

Add new line to correct alignment.

### Are these changes tested?

Yes. I have verified the changes by building the documentation.

### Are there any user-facing changes?

Yes. the updated documentation will be visible to users.

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>