Wrong call in README example

The python API example given in the README file should be :
```
convert.bubble_to_dot('path/to/bubble.bbl', dotfile='path/to/dot.dot')
```
instead of
```
convert.to_dot(open('path/to/bubble.lp'), dotfile='path/to/dot.dot')
```

Also there is several references to bubble files but with `lp` extension ?!