Update docs to use jsonRDD instead of wrong jsonRdd.

Author: Grega Kespret <grega.kespret@gmail.com>

Closes apache#2479 from gregakespret/patch-1 and squashes the following commits:

dd6b90a [Grega Kespret] Update docs to use jsonRDD instead of wrong jsonRdd.

docs/sql-programming-guide.md

@@ -605,7 +605,7 @@ Spark SQL can automatically infer the schema of a JSON dataset and load it as a
 This conversion can be done using one of two methods in a SQLContext:
 
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
-* `jsonRdd` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
+* `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
 {% highlight scala %}
 // sc is an existing SparkContext.
@@ -643,7 +643,7 @@ Spark SQL can automatically infer the schema of a JSON dataset and load it as a
 This conversion can be done using one of two methods in a JavaSQLContext :
 
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
-* `jsonRdd` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
+* `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
 {% highlight java %}
 // sc is an existing JavaSparkContext.
@@ -681,7 +681,7 @@ Spark SQL can automatically infer the schema of a JSON dataset and load it as a
 This conversion can be done using one of two methods in a SQLContext:
 
 * `jsonFile` - loads data from a directory of JSON files where each line of the files is a JSON object.
-* `jsonRdd` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
+* `jsonRDD` - loads data from an existing RDD where each element of the RDD is a string containing a JSON object.
 
 {% highlight python %}
 # sc is an existing SparkContext.