out-dated documentation for calling atmospheric model

There is a slightly out-dated instruction on Challenge #2 within the [atmosphere tutorial](https://github.com/smrt-model/tutorials/blob/master/06_substrate_atmosphere/03_atmosphere.ipynb): "Include atmosphere in model run (medium = atmos + snow)".

As of SMRT v1.3, `atmos` should be included via `snowpack.atmosphere=atmos`.