Fix typo in ecmult_const_impl.h
Fix typo in ecmult_const_impl.h

Fix small typo in the reference given for the wNAF method
afk11/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,ElementsProject/secp256k1-zkp,evoskuil/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,afk11/secp256k1,bitcoin-core/secp256k1,peterdettman/secp256k1,evoskuil/secp256k1,afk11/secp256k1,bitcoin/secp256k1,afk11/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,afk11/secp256k1,bitcoin/secp256k1