[SPARK-36341][SQL] Aggregated Metrics by Executor link should wrap <a> with <h4>

### What changes were proposed in this pull request?
In current stage page, when we move mouse to the title of `Aggregated Metrics by Executor`, the underline is blocked.
![image](https://user-images.githubusercontent.com/46485123/127449431-8897e648-c7cd-4bff-adb1-f43347c381b1.png)

For completed jobs, the code is
![image](https://user-images.githubusercontent.com/46485123/127449708-d6a7ccea-a1b5-4251-9648-38c5107457f8.png)

After this pr
![image](https://user-images.githubusercontent.com/46485123/127449465-6631076e-2eb4-41eb-a5ef-6c3c8be08d87.png)

### Why are the changes needed?

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

Closes #33571 from AngersZhuuuu/SPARK-36341.

Authored-by: Angerszhuuuu <angers.zhu@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>

core/src/main/resources/org/apache/spark/ui/static/stagespage-template.html

@@ -31,10 +31,10 @@ limitations under the License.
             </tbody>
         </table>
     </div>
-    <a id="aggregatedMetrics" class="collapse-table">
+    <h4 id="aggregatedMetrics" class="collapse-table">
         <span class="expand-input-rate-arrow arrow-closed" id="arrowtoggle2"></span>
-        <h4 class="title-table">Aggregated Metrics by Executor</h4>
-    </a>
+        <a class="title-table">Aggregated Metrics by Executor</a>
+    </h4>
     <br>
     <div class="container-fluid d-none" id="toggle-aggregatedMetrics">
         <table id="summary-executor-table" class="table table-striped compact table-dataTable cell-border">