docs: update version number in example on readme (#59)

## what

## why
- Bring copy/paste example to do date with module versions published on
TF registries.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Refreshed Terraform usage examples with improved formatting and
alignment for readability.
* Updated the ssm-agent example to reference the latest major version
(1.8.0) and clarified field ordering.
* Adjusted the dynamic-subnets example indentation and spacing without
changing behavior.
* Enhancements make copy-paste setup clearer and reduce confusion around
versioning and configuration layout.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: westonplatter

README.md

@@ -29,17 +29,19 @@ Big shout out to the following projects which this project uses/depends on/menti
 
 ```hcl
 module "ssm_agent" {
-  source     = "masterpointio/ssm-agent/aws"
-  version    = "0.15.1"
-  stage      = var.stage
-  namespace  = var.namespace
+  source  = "masterpointio/ssm-agent/aws"
+  version = "X.X.X"
+
+  namespace = var.namespace
+  stage     = var.stage
+
   vpc_id     = module.vpc.vpc_id
   subnet_ids = module.subnets.private_subnet_ids
 }
 
 module "vpc" {
   source  = "cloudposse/vpc/aws"
-  version = "2.1.0"
+  version = "X.X.X"
 
   namespace = var.namespace
   stage     = var.stage
@@ -50,8 +52,9 @@ module "vpc" {
 }
 
 module "subnets" {
-  source    = "cloudposse/dynamic-subnets/aws"
-  version   = "2.3.0"
+  source  = "cloudposse/dynamic-subnets/aws"
+  version = "X.X.X"
+
   namespace = var.namespace
   stage     = var.stage