[KYUUBI #5951] [DOC] Improve doc for Session Conf Advisor

# 🔍 Description
## Issue References 🔗

This pull request is to improve doc for Session Conf Advisor

## Describe Your Solution 🔧

improve doc for Session Conf Advisor

## Types of changes 🔖

- [ ] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Test Plan 🧪

#### Behavior Without This Pull Request ⚰️

#### Behavior With This Pull Request 🎉

#### Related Unit Tests

---

# Checklist 📝

- [ ] This patch was not authored or co-authored using [Generative Tooling](https://www.apache.org/legal/generative-tooling.html)

**Be nice. Be informative.**

Closes #5951 from zhaohehuhu/Improvement-0108.

Closes #5951

9f2c662 [hezhao2] Improve doc for Session Conf Advisor

Authored-by: hezhao2 <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>

docs/extensions/server/configuration.rst

@@ -53,8 +53,8 @@ We have a custom class ``CustomSessionConfAdvisor``:
 
 .. code-block:: java
 
-   @Override
-   public class CustomSessionConfAdvisor {
+   public class CustomSessionConfAdvisor implements SessionConfAdvisor {
+     @Override
      Map<String, String> getConfOverlay(String user, Map<String, String> sessionConf) {
        if ("uly".equals(user)) {
          return Collections.singletonMap("spark.driver.memory", "1G");