DOC: Clean up reference to cluster object (#2664)
DOC: Clean up reference to cluster object (#2664)

The current docs would overwrite the cluster objects, so it needs to get its own variable name.
As the Client is usually named `c`, I changed the cluster object to the name `cluster`
blaze/distributed,dask/distributed,blaze/distributed,mrocklin/distributed,mrocklin/distributed,mrocklin/distributed,dask/distributed,dask/distributed,dask/distributed