Adds missing line to the model slice's help output

A user recently asked for help with the syntax for getting the list of available model templates via the Razor IRC channel.  After going back and looking at the `razor model --help` output, it became apparent that there was a line missing in this output.  This pull request adds that missing line into that output (which shows users how to get the list of model templates via a `razor model template` or `razor model templates` command).