[SPARK-8927] [DOCS] Format wrong for some config descriptions

A couple descriptions were not inside `<td></td>` and were being displayed immediately under the section title instead of in their row.

Author: Jonathan Alter <[email protected]>

Closes #7292 from jonalter/docs-config and squashes the following commits:

5ce1570 [Jonathan Alter] [DOCS] Format wrong for some config descriptions

(cherry picked from commit 28fa01e)
Signed-off-by: Sean Owen <[email protected]>

Author: jonalter

docs/configuration.md

@@ -1007,9 +1007,9 @@ Apart from these, the following properties are also available, and may be useful
 <tr>
   <td><code>spark.rpc.numRetries</code></td>
   <td>3</td>
+  <td>
     Number of times to retry before an RPC task gives up.
     An RPC task will run at most times of this number.
-  <td>
   </td>
 </tr>
 <tr>
@@ -1029,8 +1029,8 @@ Apart from these, the following properties are also available, and may be useful
 <tr>
   <td><code>spark.rpc.lookupTimeout</code></td>
   <td>120s</td>
-    Duration for an RPC remote endpoint lookup operation to wait before timing out.
   <td>
+    Duration for an RPC remote endpoint lookup operation to wait before timing out.  
   </td>
 </tr>
 </table>