[SPARK-19460][SPARKR] Update dataset used in R documentation, examples to reduce warning noise and confusions

## What changes were proposed in this pull request?

Replace `iris` dataset with `Titanic` or other dataset in example and document.

## How was this patch tested?

Manual and existing test

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #17032 from wangmiao1981/example.