[SPARK-35405][DOC] Submitting Applications documentation has outdated…

… information about K8s client mode support

### What changes were proposed in this pull request?
[Submitting Applications doc](https://spark.apache.org/docs/latest/submitting-applications.html#master-urls) has outdated information about K8s client mode support.
It still says "Client mode is currently unsupported and will be supported in future releases".
![image](https://user-images.githubusercontent.com/31073930/118268920-b5b51580-b4c6-11eb-8eed-975be8d37964.png)

Whereas it's already supported and [Running Spark on Kubernetes doc](https://spark.apache.org/docs/latest/running-on-kubernetes.html#client-mode) says that it's supported started from 2.4.0 and has all needed information.
![image](https://user-images.githubusercontent.com/31073930/118268947-bd74ba00-b4c6-11eb-98d5-37961327642f.png)

Changes:
![image](https://user-images.githubusercontent.com/31073930/118269179-12b0cb80-b4c7-11eb-8a37-d9d301bbda53.png)

JIRA: https://issues.apache.org/jira/browse/SPARK-35405

### Why are the changes needed?
Outdated information in the doc is misleading

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

### How was this patch tested?
Documentation changes

Closes apache#32551 from o-shevchenko/SPARK-35405.

Authored-by: Oleksandr Shevchenko <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

docs/submitting-applications.md

@@ -181,7 +181,7 @@ The master URL passed to Spark can be in one of the following formats:
         The cluster location will be found based on the <code>HADOOP_CONF_DIR</code> or <code>YARN_CONF_DIR</code> variable.
 </td></tr>
 <tr><td> <code>k8s://HOST:PORT</code> </td><td> Connect to a <a href="running-on-kubernetes.html">Kubernetes</a> cluster in
-        <code>cluster</code> mode. Client mode is currently unsupported and will be supported in future releases.
+        <code>client</code> or <code>cluster</code> mode depending on the value of <code>--deploy-mode</code>.
         The <code>HOST</code> and <code>PORT</code> refer to the <a href="https://kubernetes.io/docs/reference/generated/kube-apiserver/">Kubernetes API Server</a>.
         It connects using TLS by default. In order to force it to use an unsecured connection, you can use
         <code>k8s://http://HOST:PORT</code>.