Updated README
Updated README

Fixed bad git repository reference.
afk11/secp256k1-php,afk11/secp256k1-php,afk11/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,afk11/secp256k1-php,afk11/secp256k1-php