updated a SOS tag in the EC2 Yaml file

Author: scmacdon

.doc_gen/metadata/ec2_metadata.yaml

@@ -1494,7 +1494,7 @@ ec2_DescribeInstances:
           excerpts:
             - description:
               snippet_tags:
-                - ec2.java2.describe_instances.main
+                - ec2.java2.scenario.describe_instance.main
     .NET:
       versions:
         - sdk_version: 3