[SPARK-18001][DOCUMENT] fix broke link to SparkDataFrame

## What changes were proposed in this pull request?

In http://spark.apache.org/docs/latest/sql-programming-guide.html, Section "Untyped Dataset Operations (aka DataFrame Operations)"

Link to R DataFrame doesn't work that return
The requested URL /docs/latest/api/R/DataFrame.html was not found on this server.

Correct link is SparkDataFrame.html for spark 2.0

## How was this patch tested?

Manual checked.

Author: Tommy YU <tummyyu@163.com>

Closes #15543 from Wenpei/spark-18001.