Docs: Update custom-graph.rst [skip ci] (#4296)
Docs: Update custom-graph.rst [skip ci] (#4296)

This PR updates `custom-graph.rst` with some punctuation and naming corrections/improvements, and a few sentences were slightly changed to improve clarity
jakirkham/dask,ContinuumIO/dask,dask/dask,jcrist/dask,mrocklin/dask,jcrist/dask,blaze/dask,blaze/dask,dask/dask,ContinuumIO/dask,mrocklin/dask,jakirkham/dask