Update shell_plus.rst

I noticed the options to set a python shell have changed:

```
$ ./manage.py shell_plus -h
Usage: ./manage.py shell_plus [options]

Like the 'shell' command but autoloads the models of all installed Django apps.

Options:
  -v VERBOSITY, --verbosity=VERBOSITY
                        Verbosity level; 0=minimal output, 1=normal output,
                        2=verbose output, 3=very verbose output
  --settings=SETTINGS   The Python path to a settings module, e.g.
                        "myproject.settings.main". If this isn't provided, the
                        DJANGO_SETTINGS_MODULE environment variable will be
                        used.
  --pythonpath=PYTHONPATH
                        A directory to add to the Python path, e.g.
                        "/home/djangoprojects/myproject".
  --traceback           Raise on exception
  --no-color            Don't colorize the command output.
  --plain               Tells Django to use plain Python, not BPython nor
                        IPython.
  --bpython             Tells Django to use BPython, not IPython.
  --ipython             Tells Django to use IPython, not BPython.
  --notebook            Tells Django to use IPython Notebook.
  --use-pythonrc        Tells Django to execute PYTHONSTARTUP file (BE
                        CAREFULL WITH THIS!)
  --print-sql           Print SQL queries as they're executed
  --dont-load=DONT_LOAD
                        Ignore autoloading of some apps/models. Can be used
                        several times.
  --quiet-load          Do not display loaded models messages
  --version             show program's version number and exit
  -h, --help            show this help message and exit
```

On a side note, SHELL_PLUS_PRE_IMPORTS is not doing anything in 1.3.8 using the example (this should throw errors as I don't have any of those modules, but it does nothing)
Merge pull request #527 from razzius/patch-1

Update shell_plus.rst