Docs: Update futures.rst (#4262)
Docs: Update futures.rst (#4262)

This PR updates `futures.rst` with some punctuation and typo corrections. However, I've found a couple of issues that may need some special oversight.

Two important notes:

- In the **Actors** section, some note commands were either not properly defined or were hidden on purpose, but I couldn't tell which one was it. If it was the later, then I need to revert the changes I made in order to hide the note messages.
- The last two sentences in the **Asynchronous Operation** subsection of the **Actors** section require a re-write (I've highlighted them using **  **). I'm not sure if they are supposed to be a single sentence or not, but it seems like someone forgot about them when refactoring them. Someone should take a look at them to infer their real meaning.
ContinuumIO/dask,blaze/dask,jakirkham/dask,ContinuumIO/dask,mrocklin/dask,jcrist/dask,jakirkham/dask,mrocklin/dask,dask/dask,blaze/dask,dask/dask,jcrist/dask