Chapter 12, exercise 12 - mistake in the denominator

Hi.
Thank you very much for writing an excellent book, and maintaining the notebooks.
In exercise 12 of chapter 12, in the call method, I believe the return statement should state

`return self.alpha * (X - mean) / (tf.sqrt(variance) + self.eps) + self.beta`

in order to match alpha.*(X-mu)/(sigma + eps) + beta.
The current implementation is:

```
def call(self, X):
mean, variance = tf.nn.moments(X, axes=-1, keepdims=True)
return self.alpha * (X - mean) / (tf.sqrt(variance + self.eps)) + self.beta
```