[SPARK-32525][DOCS] The layout of monitoring.html is broken

### What changes were proposed in this pull request?

This PR fixes the layout of monitoring.html broken after SPARK-31566(apache#28354).
The cause is there are 2 `<td>` tags not closed in `monitoring.md`.

### Why are the changes needed?

This is a bug.

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

No.

### How was this patch tested?

Build docs and the following screenshots are before/after.

* Before fixed
![broken-doc](https://user-images.githubusercontent.com/4736016/89257873-fba09b80-d661-11ea-90da-06cbc0783011.png)

* After fixed.
![fixed-doc2](https://user-images.githubusercontent.com/4736016/89257910-0fe49880-d662-11ea-9a85-7a1ecb1d38d6.png)

Of course, the table is still rendered correctly.
![fixed-doc1](https://user-images.githubusercontent.com/4736016/89257948-225ed200-d662-11ea-80fd-d9254b44d4a0.png)

Closes apache#29345 from sarutak/fix-monitoring.md.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Gengliang Wang <gengliang.wang@databricks.com>

docs/monitoring.md

@@ -573,6 +573,7 @@ can be identified by their `[attempt-id]`. In the API listed below, when running
     <code>?planDescription=[true (default) | false]</code> enables/disables Physical <code>planDescription</code> on demand when Physical Plan size is high.
     <br>
     <code>?offset=[offset]&length=[len]</code> lists queries in the given range.
+    </td>
   </tr>
   <tr>
     <td><code>/applications/[app-id]/sql/[execution-id]</code></td>
@@ -581,6 +582,7 @@ can be identified by their `[attempt-id]`. In the API listed below, when running
     <code>?details=[true (default) | false]</code> lists/hides metric details in addition to given query details.
     <br>
     <code>?planDescription=[true (default) | false]</code> enables/disables Physical <code>planDescription</code> on demand for the given query when Physical Plan size is high.
+    </td>
   </tr>
   <tr>
     <td><code>/applications/[app-id]/environment</code></td>