[Inference API] Fix comment in ElasticInferenceServiceFeature to mention new way of enabling elastic inference service feature (#120274)

Author: timgrein

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/elastic/ElasticInferenceServiceFeature.java

@@ -11,7 +11,7 @@
 
 /**
  * Elastic Inference Service (EIS) feature flag. When the feature is complete, this flag will be removed.
- * Enable feature via JVM option: `-Des.eis_feature_flag_enabled=true`.
+ * Enable feature via JVM option: `-Des.elastic_inference_service_feature_flag_enabled=true`.
  */
 public class ElasticInferenceServiceFeature {