[SPARK-18797][SPARKR] Update spark.logit in sparkr-vignettes

## What changes were proposed in this pull request?
spark.logit is added in 2.1. We need to update spark-vignettes to reflect the changes. This is part of SparkR QA work.

## How was this patch tested?

Manual build html. Please see attached image for the result.
![test](https://cloud.githubusercontent.com/assets/5033592/21032237/01b565fe-bd5d-11e6-8b59-4de4b6ef611d.jpeg

)

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

Closes #16222 from wangmiao1981/veg.

(cherry picked from commit 2aa16d03)
Signed-off-by: Xiangrui Meng <meng@databricks.com>