Fix wrong var name in HCL guide

[antoineco]

The example below taken from the guide

variable "network_numbers" {
  type = list(number)
}

In this example, the variable "network_numbers" block makes var.base_network_addr available to expressions,

should instead read:

In this example, the `variable "network_numbers"` block makes
- `var.base_network_addr`
+ `var.network_numbers`
available to expressions,

[antoineco]

cc @jbardin @apparentlymart (I think you are the most active maintainers of this repo?)

[radeksimko]

Good catch, thank you for the PR!