SPARK-1976: fix the misleading part in  streaming docs

Spark streaming requires at least two working threads, but the document gives the example like

import org.apache.spark.api.java.function._
import org.apache.spark.streaming._
import org.apache.spark.streaming.api._
// Create a StreamingContext with a local master
val ssc = new StreamingContext("local", "NetworkWordCount", Seconds(1))
http://spark.apache.org/docs/latest/streaming-programming-guide.html