bitcoin_hdwallet example error

[Yinhuan Yuan]

The key should be changed to extended_key. Otherwise, it will fail to compile.
let (extended_key, derivation) = key_chain.derive_private_key("m/1H/0".into()).expect("derive ExtendedPrivKey");
let key = BitcoinPrivKey { network: BitcoinNetwork::MainNet, derivation, extended_key, };