[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 #20312 from brandonJY/patch-2.

(cherry picked from commit 6121e91)
Signed-off-by: Sean Owen <[email protected]>

Author: brandonJY

docs/structured-streaming-kafka-integration.md

@@ -59,7 +59,7 @@ 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")
@@ -68,7 +68,7 @@ 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")
@@ -77,7 +77,7 @@ 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")
@@ -169,7 +169,7 @@ 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")
@@ -178,7 +178,7 @@ 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")
@@ -189,7 +189,7 @@ 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")