Update crypto.md

Only documentation is changed: latin1 -> utf8

referred : https://github.com/nodejs/node/commit/b010c8716498dca398e61c388859fea92296feb3

function toBuf(str, encoding) {
  if (typeof str === 'string') {
    if (encoding === 'buffer' || !encoding)
      encoding = 'utf8';
    return new Buffer(str, encoding);
  }
  return str;
}