[MINOR][SQL] The comment of Class ExchangeCoordinator exist a typing and context error

## What changes were proposed in this pull request?

The given example in the comment of Class ExchangeCoordinator is exist four post-shuffle partitions,but the current comment is “three”.

## How was this patch tested?

Author: lufei <lu.fei80@zte.com.cn>

Closes #19028 from figo77/SPARK-21816.