Small fix for executor example in README.md

[dee-me-tree-or-love]

In the README example, there was a problem with using dask.delayed since delayed is imported from dask:

from math import sqrt
from dask.distributed import Executor
from dask import delayed
# no import of `dask`
...
tasks = [dask.delayed(sqrt)(i) for i in range(100)] 
# `dask` is not declared

tasks = [delayed(sqrt)(i) for i in range(100)]
# works
...

I know that's just a little picky note, but I hope it maybe helps ✌️