website: fix misspelled argument name for aws_db_instance resource

When using the `subnet_group_name` argument as documented we have the following error : 
```
There are warnings and/or errors related to your configuration. Please
fix these before continuing.

Errors:

  * 'aws_db_instance.ha-rds' error: Unknown configuration: subnet_group_name
```

Indeed [the validator](https://github.com/hashicorp/terraform/blob/master/builtin/providers/aws/resource_aws_db_instance.go#L342) is waiting for `db_subnet_group_name`