[SPARK-23668][K8S] Added missing config property in running-on-kubernetes.md

## What changes were proposed in this pull request?
PR #20811 introduced a new Spark configuration property `spark.kubernetes.container.image.pullSecrets` for specifying image pull secrets. However, the documentation wasn't updated accordingly. This PR adds the property introduced into running-on-kubernetes.md.

## How was this patch tested?
N/A.

foxish mccheah please help merge this. Thanks!

Author: Yinan Li <[email protected]>

Closes #21480 from liyinan926/master.

Author: liyinan926

docs/running-on-kubernetes.md

@@ -327,6 +327,13 @@ specific to Spark on Kubernetes.
     Container image pull policy used when pulling images within Kubernetes.
   </td>
 </tr>
+<tr>
+  <td><code>spark.kubernetes.container.image.pullSecrets</code></td>
+  <td><code></code></td>
+  <td>
+    Comma separated list of Kubernetes secrets used to pull images from private image registries.
+  </td>
+</tr>
 <tr>
   <td><code>spark.kubernetes.allocation.batch.size</code></td>
   <td><code>5</code></td>