[Docs] Add missing <code> tags (minor)

These configs looked inconsistent from the rest.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1936 from andrewor14/docs-code and squashes the following commits:

15f578a [Andrew Or] Add <code> tag

docs/configuration.md

@@ -562,7 +562,7 @@ Apart from these, the following properties are also available, and may be useful
   </td>
 </tr>
 <tr>
-    <td>spark.hadoop.validateOutputSpecs</td>
+    <td><code>spark.hadoop.validateOutputSpecs</code></td>
     <td>true</td>
     <td>If set to true, validates the output specification (e.g. checking if the output directory already exists)
     used in saveAsHadoopFile and other variants. This can be disabled to silence exceptions due to pre-existing
@@ -570,7 +570,7 @@ Apart from these, the following properties are also available, and may be useful
     previous versions of Spark. Simply use Hadoop's FileSystem API to delete output directories by hand.</td>
 </tr>
 <tr>
-    <td>spark.executor.heartbeatInterval</td>
+    <td><code>spark.executor.heartbeatInterval</code></td>
     <td>10000</td>
     <td>Interval (milliseconds) between each executor's heartbeats to the driver.  Heartbeats let
     the driver know that the executor is still alive and update it with metrics for in-progress