[JOSE] xarray module is imported as xr but tied to call as xarray

[RobTheOceanographer]

In lesson 07-vectorisation the first call to xarray.open_dataset has a syntax error as the xarray module is imported as xr:
Change xarray.open_dataset to xr.open_dataset ?

CC: openjournals/jose-reviews#37

[RobTheOceanographer]

there is actually a second instance of this further down in this lesson that need addressing too.

[DamienIrving]

Thanks for spotting this error, @RobTheOceanographer.

I've corrected both calls to xr.open_dataset (see carpentrieslab/python-aos-lesson@d43e966).