Add required provider block to the terrafom local exercise

[ahmadpiran]

This pull request addresses the issue here in the Terraform exercise where the main.tf file is missing a required_providers block. Without this block, running terraform init results in the following error:

Error: Invalid provider registry host
The host "registry.terraform.io" given in provider source address "registry.terraform.io/hashicorp/local" does not offer a
Terraform provider registry.

The fix adds a terraform block with the required provider details, ensuring compatibility with Terraform 0.13 and later versions.