Docs: Update dataframe-extend.rst (#4231)
Docs: Update dataframe-extend.rst (#4231)

This PR updates `dataframe-extend.rst` with a few punctuation and naming convention corrections (minor stuff)
jakirkham/dask,mrocklin/dask,ContinuumIO/dask,blaze/dask,ContinuumIO/dask,blaze/dask,dask/dask,dask/dask,jcrist/dask,mrocklin/dask,jcrist/dask,jakirkham/dask