Add example to RewriteRule docstring
Add example to RewriteRule docstring

Add example using callable right-hand-side for a rewrite rule.

wiso/dask,dask/dask,ContinuumIO/dask,mikegraham/dask,pombredanne/dask,gameduell/dask,blaze/dask,mraspaud/dask,jcrist/dask,esc/dask,clarkfitzg/dask,dask/dask,marianotepper/dask,chrisbarber/dask,jakirkham/dask,PhE/dask,cpcloud/dask,blaze/dask,marianotepper/dask,freeman-lab/dask,vikhyat/dask,jayhetee/dask,jayhetee/dask,ContinuumIO/dask,mrocklin/dask,cowlicks/dask,clarkfitzg/dask,ssanderson/dask,hainm/dask,mraspaud/dask,minrk/dask,minrk/dask,pombredanne/dask,vikhyat/dask,ssanderson/dask,mrocklin/dask,wiso/dask,jcrist/dask,freeman-lab/dask,simudream/dask,PhE/dask,jakirkham/dask,hainm/dask,esc/dask,simudream/dask