little typo in code

Hi, 
I guess there may be some problem with the comment in [this line](https://github.com/twhui/SRGAN-pyTorch/blame/master/models/modules/vgg_feat.py#L36).
If the input range is [-1, 1],  the mean will be:
```python
# [2*0.485-1, 2*0.456-1, 2*0.406-1] 
```
rather than:
```python
# [0.485-1, 0.456-1, 0.406-1] 
```
