[SPARK-21843] testNameNote should be "(minNumPostShufflePartitions: 5)"

Signed-off-by: iamhumanbeing <iamhumanbeinggmail.com>

## What changes were proposed in this pull request?

testNameNote = "(minNumPostShufflePartitions: 3) is not correct.
it should be "(minNumPostShufflePartitions: " + numPartitions + ")" in ExchangeCoordinatorSuite

## How was this patch tested?

unit tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: iamhumanbeing <iamhumanbeing@gmail.com>

Closes #19058 from iamhumanbeing/testnote.