Corrected reference to blockwise to refer to apply_gufunc instead

[TomNicholas]

I noticed that the apply_ufunc tutorial notebook says that xarray.apply_ufunc uses dask.array.blockwise, but that's no longer true as of PR #4060 .

• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[TomNicholas]

I didn't intend to change those .ipynb version and formatting options though...

[dcherian]

thanks!