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

Author: CodingCat <zhunansjtu@gmail.com>

Closes #924 from CodingCat/master and squashes the following commits:

bb89f20 [CodingCat] update streaming docs

(cherry picked from commit 41bfdda)
Signed-off-by: Reynold Xin <rxin@apache.org>

docs/streaming-programming-guide.md

@@ -80,7 +80,8 @@ 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))
+// Spark Streaming needs at least two working thread
+val ssc = new StreamingContext("local[2]", "NetworkWordCount", Seconds(1))
 {% endhighlight %}
 
 Using this context, we then create a new DStream