Update the admin register documentation to return a real user ID

Presumably this is the intention anyways. I've also updated the domain part to be something more along the lines of what people might expect.

docs/admin_api/register_api.rst

@@ -33,7 +33,7 @@ As an example::
 
   < {
      "access_token": "token_here",
-     "user_id": "@pepper_roni@test",
+     "user_id": "@pepper_roni:localhost",
      "home_server": "test",
      "device_id": "device_id_here"
     }