Dataframe sample method docstring fix (#3566)
Dataframe sample method docstring fix (#3566)

* Removes "optional" from frac parameter

* Adds space to fix code syntax

jcrist/dask,blaze/dask,ContinuumIO/dask,dask/dask,mrocklin/dask,jakirkham/dask,jcrist/dask,dask/dask,jakirkham/dask,blaze/dask,mrocklin/dask,ContinuumIO/dask