Docs: Update array-design.rst (#4180)
Docs: Update array-design.rst (#4180)

This PR updates `array.design.rst` with minor corrections to punctuation and some wording improvements.
jakirkham/dask,jcrist/dask,mrocklin/dask,blaze/dask,mrocklin/dask,blaze/dask,dask/dask,dask/dask,jcrist/dask,ContinuumIO/dask,jakirkham/dask,ContinuumIO/dask