fix bug

defaults

@@ -1,5 +1,5 @@
 {
     "bcftools_docker": "registry.cn-shanghai.aliyuncs.com/pgx-docker-registry/bcftools:v1.9",
     "disk_size": "200",
-    "cluster_config": "OnDemand bcs.a2.3xlarge img-ubuntu-vpc"
+    "cluster_config": "OnDemand bcs.a2.large img-ubuntu-vpc"
 }