Changed the area GeneratingFunction. Here you may findoutdated documentation (in the middle of the document).
, . RSolve . :
GeneratingFunction[
RSolve[{f[n + 2] == f[n + 1] + f[n], f[0] == 0, f[1] == 1}, f[n], n],
n, x]
{{GeneratingFunction[f[n], n, x] -> -(x/(-1 + x + x^2))}}