[docs] Trivial os.path documentation bug

Tiny typo: in the Python 3.3 os.makedirs documentation at http://docs.python.org/3.3/library/os.html#os.makedirs, the sentence saying "If exist*s*_ok is False (the default), an OSError is raised..." refers to the keyword argument as "exists_ok" (with an "s"), while the other parts refer to the keyword parameter as "exist_ok" (without the "s"). Thanks for providing and maintaining all the excellent documentation. Neil