Update parallel magic doc

I added the command to import `IPython.parallel` and imported `Client` from it to be consistent with this page:

http://ipython.org/ipython-doc/dev/parallel/parallel_multiengine.html#creating-a-directview-instance

It's pretty trivial, but people just starting with ipython parrallel may not have seen the appropriate import.

I checked, and `parallel.`... is not used elsewhere in the page, so it shouldn't break other commands to change the import.