Update 'defaults'

defaults

@@ -3,7 +3,7 @@
     "ref_dir": "oss://pgx-reference-data/GRCh38.d1.vd1/",
     "disk_size": "200",
     "docker": "registry.cn-shanghai.aliyuncs.com/pgx-docker-registry/bamreadcount:v0.8.0",
-    "cluster_config": "OnDemand bcs.b2.3xlarge img-ubuntu-vpc",
+    "cluster_config": "OnDemand bcs.a2.large img-ubuntu-vpc",
     "min_base_quality": "0",
     "min_mapping_quality": "0",
     "count_docker": "registry.cn-shanghai.aliyuncs.com/pgx-docker-registry/coverage_count:v1.0"