[trivial][bc-azure-216] Fix example to reference appropriate resource type

[tdg5]

The example in this documentation should be referencing an azurerm_firewall resource not an azurerm_firewall_policy resource.

Here's the current doc should you care to reference: https://docs.prismacloud.io/en/enterprise-edition/policy-reference/azure-policies/azure-networking-policies/bc-azure-216