What is wrong? I'd start by pointing out that this is NOT Newton's method as you wrote it.
Newton (or Newton/Raphson if you wish) has
xnew = xold - f(xold)/f'(xold)
So, if f(x) = x^5 - y, then what should you have written there? So, y is the value you want to compute a root of. Clearly, the 5th root of y will produce zero in f, as I wrote it.
So now you need to write the Newton iteration to use fas I wrote it, as well as the derivative of f.