[SPARKR][DOCS][MINOR] Use consistent names in rollup and cube examples

## What changes were proposed in this pull request?

Rename `carsDF` to `df` in SparkR `rollup` and `cube` examples.

## How was this patch tested?

Manual tests.

Author: zero323 <zero323@users.noreply.github.com>

Closes #17988 from zero323/cube-docs.