SPARK-4276 fix for two working thread

Spark streaming requires at least two working threads.But example in spark/examples/src/main/scala/org/apache/spark/examples/streaming/NetworkWordCount.scala 
has
// Create the context with a 1 second batch size
val sparkConf = new SparkConf().setAppName("NetworkWordCount")
val ssc = new StreamingContext(sparkConf, Seconds(1))
which creates only 1 thread.
It should have atleast 2 threads:
http://spark.apache.org/docs/latest/streaming-programming-guide.html