[SPARKR][MINOR] Fix LDA doc

## What changes were proposed in this pull request?

This PR tries to fix the name of the `SparkDataFrame` used in the example. Also, it gives a reference url of an example data file so that users can play with.

## How was this patch tested?

Manual test.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14853 from junyangq/SPARKR-FixLDADoc.