[MINOR][DOCS] Update running-on-kubernetes.md

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

Fix executor container name typo. `executor` should be `spark-kubernetes-executor`.

### Why are the changes needed?

The Executor pod container name the users actually get from their Kubernetes clusters is different from that described in the documentation.

For example, below is what a user get from an executor pod.
```
Containers:
  spark-kubernetes-executor:
    Container ID:  docker://aaaabbbbccccddddeeeeffff
    Image:         <imagename>
    Image ID:      docker-pullable://0000.dkr.ecr.us-east-0.amazonaws.com/spark
    Port:          7079/TCP
    Host Port:     0/TCP
    Args:
      executor
    State:          Running
      Started:      Thu, 28 May 2020 05:54:04 -0700
    Ready:          True
    Restart Count:  0
    Limits:
      memory:  16Gi
```

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

Document change.

### How was this patch tested?

N/A

Closes #28862 from yuj/patch-1.

Authored-by: James Yu <yuj@users.noreply.github.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

docs/running-on-kubernetes.md

@@ -1341,7 +1341,7 @@ The following affect the driver and executor containers. All other containers in
   <td>See description</td>
   <td>
     The container name will be assigned by spark ("spark-kubernetes-driver" for the driver container, and
-    "executor" for each executor container) if not defined by the pod template. If the container is defined by the
+    "spark-kubernetes-executor" for each executor container) if not defined by the pod template. If the container is defined by the
     template, the template's name will be used.
   </td>
 </tr>