Fix *args as positional arguments

[mwtoews]

This PR fixes the wording to describe *args as positional arguments, and adds a full stop at the end.

[stefanv]

Thanks for highlighting this confusing phrasing, @mwtoews.

Since this example is entirely made up, I was wondering if we can't come up with something more intuitive. E.g.:

def red_print(*args, **kwargs):
   """Print text in red.

   Parameters
   ----------
   *args
       Positional arguments, passed to `print`.
   **kwargs
       Keyword arguments, passed to `print`.
   ...

This is not the best example, but it's late :) Something more computational would be good.

I think this was the original goal with the metric usage in **kwargs, but it doesn't really come across.