DOC: minor fix in extract docstring
@jorisvandenbossche pls review and merge!
 @danielballan minor issue here: http://pandas.pydata.org/pandas-docs/dev/basics.html#vectorized-string-methods (scroll down a bit), I think just need `.str` accessor on the examples....
 True. Fixed.
 OK, and the rest is looking good. So I can merge my first PR
Thanks!
