updated rackspace docs

Rackspace have changed the names by adding a space and removing the word "Instance" in their server flavours names, to bring inline with new names such as `4 GB Performance` available in some locations.

This causes salt-cloud to raise a SaltCloudNotFound exception with error `Could not find size 512MB Standard Instance: The specified size could not be found."`

`512MB Standard Instance` has become `512 MB Standard` etc.

Corrected the example docs to reflect this.

Also added HKG region to the list of valid compute regions, confirmed it builds there with `OPENSTACK.compute_region: HKG` set.