Incorrect import in python_magnetrun/README.md (Loading Data section)

[ValentinLevchenko]

Current description:

### Loading data
**Single Pupitre file (`.txt`):**
```python
from python_magnetrun.magnetdata import MagnetData
...
```

However, using this import leads to the following error:
ImportError: cannot import name 'MagnetData' from 'python_magnetrun.magnetdata'

Suggested fix:

### Loading data
**Single Pupitre file (`.txt`):**
```python
from python_magnetrun.magnetdata import load_magnetdata
...
```