Documentation for IPython backend - set_options needs the client instance method

I was originally trying the following based on the set option pattern from http://dask.pydata.org/en/latest/scheduler-overview.html, and getting `TypeError: get() missing 1 required positional argument: 'keys'`

```python
import dask.array as da
from dask import set_options, distributed
from ipyparallel import Client
rc = Client()
rc = rc.become_distributed()
set_options(get=distributed.Client.get)
x = da.arange(100, chunks=10)
x.compute()
```

But in fact `set_options` should be given the instance method `rc.get` not the class method `distributed.Client.get`:

```python
import dask.array as da
from dask import set_options, distributed
from ipyparallel import Client
rc = Client()
rc = rc.become_distributed()
set_options(get=rc.get)
x = da.arange(100, chunks=10)
x.compute()
```