[MINOR][SPARKR] R API documentation for "coltypes" is confusing

## What changes were proposed in this pull request?

R API documentation for "coltypes" is confusing, found when working on another ticket.

Current version http://spark.apache.org/docs/2.0.0/api/R/coltypes.html, where parameters have 2 "x" which is a duplicate, and also the example is not very clear

![current](https://cloud.githubusercontent.com/assets/3925641/17386808/effb98ce-59a2-11e6-9657-d477d258a80c.png)

![screen shot 2016-08-03 at 5 56 00 pm](https://cloud.githubusercontent.com/assets/3925641/17386884/91831096-59a3-11e6-84af-39890b3d45d8.png)

## How was this patch tested?

Tested manually on local machine. And the screenshots are like below:

![screen shot 2016-08-07 at 11 29 20 pm](https://cloud.githubusercontent.com/assets/3925641/17471144/df36633c-5cf6-11e6-8238-4e32ead0e529.png)

![screen shot 2016-08-03 at 5 56 22 pm](https://cloud.githubusercontent.com/assets/3925641/17386896/9d36cb26-59a3-11e6-9619-6dae29f7ab17.png)

Author: Xin Ren <iamshrek@126.com>

Closes #14489 from keypointt/rExample.