[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.

docs/sql-programming-guide.md

@@ -140,7 +140,7 @@ As an example, the following creates a DataFrame based on the content of a JSON
 
 ## Untyped Dataset Operations (aka DataFrame Operations)
 
-DataFrames provide a domain-specific language for structured data manipulation in [Scala](api/scala/index.html#org.apache.spark.sql.Dataset), [Java](api/java/index.html?org/apache/spark/sql/Dataset.html), [Python](api/python/pyspark.sql.html#pyspark.sql.DataFrame) and [R](api/R/DataFrame.html).
+DataFrames provide a domain-specific language for structured data manipulation in [Scala](api/scala/index.html#org.apache.spark.sql.Dataset), [Java](api/java/index.html?org/apache/spark/sql/Dataset.html), [Python](api/python/pyspark.sql.html#pyspark.sql.DataFrame) and [R](api/R/SparkDataFrame.html).
 
 As mentioned above, in Spark 2.0, DataFrames are just Dataset of `Row`s in Scala and Java API. These operations are also referred as "untyped transformations" in contrast to "typed transformations" come with strongly typed Scala/Java Datasets.