doc: changes to readme

Author: jojustin

README.md

@@ -18,7 +18,7 @@
 ```hcl
 module "postgresql_db" {
   # replace master with version
-  source = "git::https://github.com/terraform-ibm-modules/terraform-ibm-icd-postgresql?ref=master"
+  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
   name              = var.name