typo in document

The default value of `retry_attempts` var is set to 0 in [variables.tf](https://github.com/terraform-module/terraform-aws-lambda/blob/5c82fad25e31a054573514a35510c82fc4735558/variables.tf#L95),
but [README](https://github.com/terraform-module/terraform-aws-lambda/blame/master/README.md#L87) says `Default to 2`