docs: Update version in README example config (#184)

Co-authored-by: Bryant Biggs <[email protected]>

Author: davidjb

README.md

@@ -24,7 +24,7 @@ Doing serverless with Terraform? Check out [serverless.tf framework](https://ser
 ```hcl
 module "notify_slack" {
   source  = "terraform-aws-modules/notify-slack/aws"
-  version = "~> 4.0"
+  version = "~> 5.0"
 
   sns_topic_name = "slack-topic"