[MINOR] Update configuration doc with appropriate prometheus sink class

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

Update configuration doc with appropriate prometheus sink class

### Why are the changes needed?

PrometheusPullModelSink class does not exist and leads to NoClassDefFound error

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

No

### How was this patch tested?

Just a doc change

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

No

Closes #388 from ashangit/nfraison/prometheus.

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

Author: ashangit

docs/configuration.md

@@ -115,8 +115,7 @@ to [Prometheus](https://prometheus.io).
 
 ```properties
 metrics.properties:|+
-  spark.metrics.conf.operator.sink.prometheus.class=org.apache.spark.kubernetes.operator.metrics.
-sink.PrometheusPullModelSink
+  spark.metrics.conf.operator.sink.prometheus.class=org.apache.spark.k8s.operator.metrics.PrometheusPullModelHandler
 ```
 
 * Install Spark Operator