Changes encryption to customer_managed_key for azurerm_fluid_relay_servers resource

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Change the azurerm_fluid_relay_servers resource's encryption argument to customer_managed_key in its documentation. This appears to be the name for the block based on the resource's schema: https://github.com/hashicorp/terraform-provider-azurerm/blob/v4.9.0/internal/services/fluidrelay/fluid_relay_servers_resource.go#L96

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