message: Fix a typo in documentation `demo.rst`
diff: @@ -132,7 +132,7 @@ just-in-time compiler.
  Color3fD output = srgb_gamma(input);
  FloatD loss = enoki::norm(output - Color3fD(.1f, .2f, .3f));
  - enoki::backward(output);
  + enoki::backward(loss);
  std::cout << enoki::gradient(input) << std::endl;
  