update docs to use flatten vs concat (#2764)
update docs to use flatten vs concat (#2764)

i am updating the docs according to https://github.com/dask/dask/pull/2402
ContinuumIO/dask,jakirkham/dask,jcrist/dask,mrocklin/dask,dask/dask,dask/dask,blaze/dask,blaze/dask,jcrist/dask,ContinuumIO/dask,mrocklin/dask,jakirkham/dask