Usage: usage instructions typo

In [Usage](https://github.com/vxfung/MatDeepLearn#usage), inside [Running your first calculation](https://github.com/vxfung/MatDeepLearn#running-your-first-calculation),

I believe the command in part 2 should change **from**:
```
python main.py --data_path=data/test_data
```
**to:**
```
python main.py --data_path=data/test_data/test_data
```