doc: changes to readme

Author: jojustin

README.md

@@ -17,6 +17,7 @@
 
 ```hcl
 module "postgresql_db" {
+  # replace main with version
   source = "git::https://github.com/terraform-ibm-modules/terraform-ibm-icd-postgresql?ref=main"
   admin_password    = var.admin_password
   resource_group_id = module.resource_group.resource_group_id