Replace the out of date super init method

According to this docs: https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/module.py#L379

We should use `super().__init__()` than `Super(NeuralNetwork, self).__init__()`

Author: yingshaoxo

beginner_source/basics/buildmodel_tutorial.py

@@ -49,7 +49,7 @@
 
 class NeuralNetwork(nn.Module):
     def __init__(self):
-        super(NeuralNetwork, self).__init__()
+        super().__init__()
         self.flatten = nn.Flatten()
         self.linear_relu_stack = nn.Sequential(
             nn.Linear(28*28, 512),