DOC: Added get_group() example to grouby.html

Added a get_group example in the Iterating through groups section of grouby.html to demonstrate selecting
single group from a DataFrameGrouBy object

Fixed typo in README
I don't think this will run correclty, as the example above is using `df.groupby(['A', 'B'])`, so you have to use eg `('bar', 'one')`