[SPARK-35746][UI] Fix taskid in the stage page task event timeline

### What changes were proposed in this pull request?
Task id is given incorrect in the timeline plot in Stage Page

### Why are the changes needed?
Map event timeline plots to correct task
**Before:**
![image](https://user-images.githubusercontent.com/23054875/121761077-81775800-cb4b-11eb-8ec6-ee71926a6549.png)

**After**
![image](https://user-images.githubusercontent.com/23054875/121761195-02ceea80-cb4c-11eb-8ce6-07bb1cca190e.png)
### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Manually tested

Closes apache#32888 from shahidki31/shahid/fixtaskid.

Authored-by: shahid <[email protected]>
Signed-off-by: Kousuke Saruta <[email protected]>

Author: shahidki31

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

@@ -355,7 +355,7 @@ private[ui] class StagePage(parent: StagesTab, store: AppStatusStore) extends We
                |'content': '<div class="task-assignment-timeline-content"
                  |data-toggle="tooltip" data-placement="top"
                  |data-html="true" data-container="body"
-                 |data-title="${s"Task " + index + " (attempt " + attempt + ")"}<br>
+                 |data-title="${s"Task " + taskInfo.taskId + " (attempt " + attempt + ")"}<br>
                  |Status: ${taskInfo.status}<br>
                  |Launch Time: ${UIUtils.formatDate(new Date(launchTime))}
                  |${