[DOCS] change to dataset for java code in structured-streaming-kafka-integration document

## What changes were proposed in this pull request?

In latest structured-streaming-kafka-integration document, Java code example for Kafka integration is using `DataFrame<Row>`, shouldn't it be changed to `DataSet<Row>`?

## How was this patch tested?

manual test has been performed to test the updated example Java code in Spark 2.2.1 with Kafka 1.0

Author: brandonJY <[email protected]>

Closes apache#20312 from brandonJY/patch-2.

Author: brandonJY

docs/structured-streaming-kafka-integration.md

@@ -61,7 +61,7 @@ df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 {% highlight java %}
 
 // Subscribe to 1 topic
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .readStream()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
@@ -70,7 +70,7 @@ DataFrame<Row> df = spark
 df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 
 // Subscribe to multiple topics
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .readStream()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
@@ -79,7 +79,7 @@ DataFrame<Row> df = spark
 df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 
 // Subscribe to a pattern
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .readStream()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
@@ -171,7 +171,7 @@ df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
 {% highlight java %}
 
 // Subscribe to 1 topic defaults to the earliest and latest offsets
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .read()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
@@ -180,7 +180,7 @@ DataFrame<Row> df = spark
 df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");
 
 // Subscribe to multiple topics, specifying explicit Kafka offsets
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .read()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")
@@ -191,7 +191,7 @@ DataFrame<Row> df = spark
 df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)");
 
 // Subscribe to a pattern, at the earliest and latest offsets
-DataFrame<Row> df = spark
+Dataset<Row> df = spark
   .read()
   .format("kafka")
   .option("kafka.bootstrap.servers", "host1:port1,host2:port2")