`azurerm_site_recovery_replicated_vm`: exported resource format mismatch

link: https://github.com/hashicorp/terraform-provider-azurerm/issues/23159

`managed_disk` is exported in this format:
```terraform
managed_disk = [{
# details removed
}]
```
which caused `azurerm` failed on `plan`

it works with this format:
```terraform
managed_disk {
# details removed
}
```