[SPARK-52028] Update README.md with YuniKorn 1.6.3

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

This PR aims to update `YuniKorn` docs with v1.6.3.

### Why are the changes needed?

Apache YuniKorn v1.6.3 was released on 2025-05-06 with one [JIRA issue](https://yunikorn.apache.org/release-announce/1.6.3).

- https://yunikorn.apache.org/release-announce/1.6.3/
    - apache/yunikorn-core#1007

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

No.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #186 from dongjoon-hyun/SPARK-52028.

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

Author: dongjoon-hyun

README.md

@@ -93,7 +93,7 @@ $ helm repo add yunikorn https://apache.github.io/yunikorn-release
 
 $ helm repo update
 
-$ helm install yunikorn yunikorn/yunikorn --namespace yunikorn --version 1.6.2 --create-namespace --set embedAdmissionController=false
+$ helm install yunikorn yunikorn/yunikorn --namespace yunikorn --version 1.6.3 --create-namespace --set embedAdmissionController=false
 ```
 
 Submit a Spark app to YuniKorn enabled cluster: