Readme example differentiation typo

[HeinOldewage]

The code only prints the value if an error occurred. The ifs should test for err == nil.

if xgrad, err := x.Grad(); err != nil {
    fmt.Printf("dz/dx: %v", xgrad)
}

if ygrad, err := y.Grad(); err != nil {
    fmt.Printf("dz/dy: %v", ygrad)
}

[chewxy]

Right you are! Could you make a PR? I'm abroad atm, but can merge pull requests