🤦 use correct feature name

Author: carlosdelest

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/synonyms/110_synonyms_invalid.yml

@@ -317,7 +317,7 @@ setup:
 ---
 "Load index with non existent synonyms set":
   - requires:
-      cluster_features: [ synonyms_set_lenient_on_non_existing ]
+      cluster_features: [ index.synonyms_set_lenient_on_non_existing ]
       reason: "requires synonyms_set_lenient_on_non_existing bug fix"
   - do:
       indices.create: