Ch4, page79 formula 4.1

I read the chinese version, the formula (4.1) is 
```
E = (1/2)sigma(yk - tk)^2
```
and the following program is also 
```
0.5 * np.sum((y-t)**2)
```
I wonder if the formula is wrong cause mean squared error on the internet is 
```
(1/n)sigma(yk-tk)^2 the coeefficient is 1/n instead of 1/2
```