README: fix vk.verify() example (not sk.verify)
README: fix vk.verify() example (not sk.verify)

Thanks to SysTheron for the catch. Closes #7.

alex/python-ecdsa,cornelinux/python-ecdsa,arnuschky/python-ecdsa,GreatFruitOmsk/python-ecdsa,romanz/python-ecdsa