[SPARK-CORE][TEST][MINOR] Fix the wrong comment in test

## What changes were proposed in this pull request?

While learning core scheduler code, I found two lines of wrong comments. This PR simply corrects the comments.

## How was this patch tested?

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #15631 from wangmiao1981/Rbug.