fix bin_to_nibbles docstring
fix bin_to_nibbles docstring

holiman/pyethereum,harlantwood/pyethereum,inzem77/pyethereum,ethermarket/pyethereum,pipermerriam/pyethereum,ebuchman/daoist_protocol,shahankhatch/pyethereum,ethereum/pyethereum,ckeenan/pyethereum,ebuchman/daoist_protocol,jnnk/pyethereum,ddworken/pyethereum,ethereum/pyethereum,karlfloersch/pyethereum,pipermerriam/pyethereum,shahankhatch/pyethereum,ethers/pyethereum,karlfloersch/pyethereum,vaporry/pyethereum