Fix UI bug introduced in #244.

The 'duration' field was incorrectly renamed to 'task time' in the table that
lists stages.

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

@@ -48,7 +48,7 @@ private[spark] class StageTable(val stages: Seq[StageInfo], val parent: JobProgr
         {if (isFairScheduler) {<th>Pool Name</th>} else {}}
         <th>Description</th>
         <th>Submitted</th>
-        <th>Task Time</th>
+        <th>Duration</th>
         <th>Tasks: Succeeded/Total</th>
         <th>Shuffle Read</th>
         <th>Shuffle Write</th>