[SPARK-20101][SQL][FOLLOW-UP] use correct config name "spark.sql.columnVector.offheap.enabled"

## What changes were proposed in this pull request?

This PR addresses [the spelling miss](https://github.com/apache/spark/pull/17436#discussion_r152189670) of the config name `spark.sql.columnVector.offheap.enabled`.
We should use `spark.sql.columnVector.offheap.enabled`.

## How was this patch tested?

Existing tests

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #19794 from kiszk/SPARK-20101-follow.