Update main.tf
Update main.tf

I recieved the error "The argument "host" is required, but no definition was found." until I added this.  This example should be updated.

```
Terraform v0.12.1
+ provider.aws v2.14.0
```
terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws