Fix small typo in docs Optimization section.
Fix small typo in docs Optimization section.

blaze/dask,ContinuumIO/dask,jakirkham/dask,vikhyat/dask,jcrist/dask,chrisbarber/dask,blaze/dask,ContinuumIO/dask,dask/dask,mrocklin/dask,mraspaud/dask,mrocklin/dask,vikhyat/dask,mikegraham/dask,jcrist/dask,mraspaud/dask,cpcloud/dask,pombredanne/dask,pombredanne/dask,jakirkham/dask,cowlicks/dask,dask/dask,gameduell/dask