fix: pass site_prefix to CloudFront module

[blimmer]

This PR updates main.tf to pass the site_prefix variable to the CloudFront module. The module references the site prefix when creating the Wordpress bucket:

terraform-aws-serverless-static-wordpress/modules/cloudfront/distribution.tf

Lines 4 to 5 in abc903d

	resource "aws_s3_bucket" "wordpress_bucket" {
	bucket = "${var.site_prefix}.${var.site_domain}"

However, site_prefix was not passed from the main module instantiation to the submodule. For me, this caused an error because I already had an S3 bucket with the www.<site_domain>.com created. www is the variable default defined here:

terraform-aws-serverless-static-wordpress/modules/cloudfront/variables.tf

Lines 6 to 10 in abc903d

	variable "site_prefix" {
	type = string
	description = "The subdomain prefix of the website domain. E.g. www"
	default = "www"
	}

Error creating S3 bucket: BucketAlreadyOwnedByYou: Your previous request to create the named bucket succeeded and you already own it.

By passing through the site prefix, with this PR, I was able to get past the error and create the bucket <custom_site_prefix>.<site_domain>.com.