Fixing config option "retained_stages" => "retainedStages".

This is a very esoteric option and it's out of sync with the style we use.
So it seems fitting to fix it for 0.9.0.

core/src/main/scala/org/apache/spark/ui/jobs/JobProgressListener.scala

@@ -33,7 +33,7 @@ import org.apache.spark.scheduler._
  */
 private[spark] class JobProgressListener(val sc: SparkContext) extends SparkListener {
   // How many stages to remember
-  val RETAINED_STAGES = sc.conf.get("spark.ui.retained_stages", "1000").toInt
+  val RETAINED_STAGES = sc.conf.get("spark.ui.retainedStages", "1000").toInt
   val DEFAULT_POOL_NAME = "default"
 
   val stageIdToPool = new HashMap[Int, String]()

docs/configuration.md

@@ -130,7 +130,7 @@ Apart from these, the following properties are also available, and may be useful
   </td>
 </tr>
 <tr>
-  <td>spark.ui.retained_stages</td>
+  <td>spark.ui.retainedStages</td>
   <td>1000</td>
   <td>
     How many stages the Spark UI remembers before garbage collecting.