[SPARK-19639][SPARKR][EXAMPLE] Add spark.svmLinear example and update vignettes

## What changes were proposed in this pull request?

We recently add the spark.svmLinear API for SparkR. We need to add an example and update the vignettes.

## How was this patch tested?

Manually run example.

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

Closes #16969 from wangmiao1981/example.