Documentation: Added screen_name parameter to get_user method

Documentation: Added screen_name parameter to get_user method

In Getting started section of the documentation, under Models sub-heading, its mentioned as:
user = api.get_user('twitter')
but it returns the error found below:
TypeError: get_user() takes 1 positional argument but 2 were given.

So it would be easier for dummies like me, if its mentioned as
user = api.get_user(screen_name='twitter')
instead.