[SPARK-21593][DOCS] Fix 2 rendering errors on configuration page

## What changes were proposed in this pull request?

Fix 2 rendering errors on configuration doc page, due to SPARK-21243 and SPARK-15355.

## How was this patch tested?

Manually built and viewed docs with jekyll

Author: Sean Owen <sowen@cloudera.com>

Closes #18793 from srowen/SPARK-21593.