[SPARK-12692][BUILD][HOT-FIX] Fix the scala style of KinesisBackedBlockRDDSuite.scala.

https://github.com/apache/spark/pull/10736 was merged yesterday and caused the master start to fail because of the style issue.

Author: Yin Huai <yhuai@databricks.com>

Closes #10742 from yhuai/fixStyle.