provide valid rexray configuration example

[charlesmims]

I believe the example provided here is invalid: https://github.com/dcos/terraform-dcos/blob/master/aws/README.md#advance-yaml-configuration

[charlesmims]

https://rexray.readthedocs.io/en/stable/user-guide/storage-providers/aws/

Note

For backwards compatibility, the driver also registers a storage driver named ec2. The use of ec2 in config files is deprecated but functional. The ec2 driver will be removed in 0.7.0, at which point all instances of ec2 in config files must use ebs instead.

I'm not sure what version of rexray is installed in dcos, but latest version is 0.11.1