Fix typo in code example (#4272)
Fix typo in code example (#4272)

dask.config.set() accepts a dictionary, not a set.
blaze/dask,jakirkham/dask,dask/dask,ContinuumIO/dask,jcrist/dask,mrocklin/dask,jakirkham/dask,jcrist/dask,mrocklin/dask,dask/dask,ContinuumIO/dask,blaze/dask