style blacklist includes hardcopy.docstring but it should be docstring.hardcopy

All's in the title.

Compare https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/style/core.py#L43 and https://github.com/matplotlib/matplotlib/blob/master/matplotlibrc.template#L553.

(To be honest I don't know why this parameter needs to be exposed...)
