## What changes were proposed in this pull request?

Updates programming guide for spark.gapply/spark.gapplyCollect.

Similar to other examples I used `faithful` dataset to demonstrate gapply's functionality.
Please, let me know if you prefer another example.

## How was this patch tested?
Existing test cases in R

Author: Narine Kokhlikyan <narine@slice.com>

Closes #14090 from NarineK/gapplyProgGuide.