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.
Can do, but to find the appropriate import, I looked at [this page](http://ipython.org/ipython-doc/dev/parallel/parallel_multiengine.html#creating-a-directview-instance), where `from IPython.parallel import Client` was used. I wanted to be consistent with that.. 

I just had a look at the other parallel docs and imports, and it looks like the consistent approach is `from IPythoon.parallel import ...`:

    ~/GitHub/ipython/docs/source/parallel$ grep -P 'import IPython.parallel|from IPython.parallel' * 
    grep: figs: Is a directory
    magics.rst:    In [1]: from IPython.parallel import Client
    magics.rst:    In [50]: %px from IPython.parallel import bind_kernel; bind_kernel()
    parallel_demos.rst:    In [1]: from IPython.parallel import Client
    parallel_intro.rst:	In [1]: from IPython.parallel import Client
    parallel_mpi.rst:    In [1]: from IPython.parallel import Client
    parallel_multiengine.rst:    In [1]: from IPython.parallel import Client
    parallel_multiengine.rst:    In [69]: from IPython.parallel import require
    parallel_multiengine.rst:    In [79]: from IPython.parallel import CompositeError
    parallel_multiengine.rst:    In [20]: from IPython.parallel import CompositeError
    parallel_task.rst:    In [1]: from IPython.parallel import Client
    parallel_task.rst:    In [9]: from IPython.parallel import depend, require, dependent
    parallel_transition.rst:    In [1]: from IPython.parallel import Client
    parallel_transition.rst:    In [11]: from IPython.parallel import Reference
    parallel_winhpc.rst:    In [1]: from IPython.parallel import *

I checked the `magics.rst` file, and an object called parallel isn't used anywhere else:

```
~/GitHub/ipython/docs/source/parallel$ grep parallel magics.rst 
.. _parallel_magics:
    In [1]: from IPython.parallel import Client
    Async parallel execution on engine(s): all
    Async parallel execution on engine(s): all
    Async parallel execution on engine(s): [0, 2]
The parallel magics are associated with a particular :class:`~.DirectView` object.
    Async parallel execution on engine(s): [0, 2]
    In [50]: %px from IPython.parallel import bind_kernel; bind_kernel()
```