[SPARK-15223][DOCS] fix wrongly named config reference

## What changes were proposed in this pull request?

The configuration setting `spark.executor.logs.rolling.size.maxBytes` was changed to `spark.executor.logs.rolling.maxSize` in 1.4 or so.

This commit fixes a remaining reference to the old name in the documentation.

Also the description for `spark.executor.logs.rolling.maxSize` was edited to clearly state that the unit for the size is bytes.

## How was this patch tested?

no tests

Author: Philipp Hoffmann <mail@philipphoffmann.de>

Closes #13001 from philipphoffmann/patch-3.