README correction

Just tried your library - very cool, thank you for creating and sharing it :)

FYI, the example in the README only works if filename is completely uppercased:
```python
pat, _ = autoguess_life_file("lifep/gun30.lif") 
```

=>

```python
pat, _ = autoguess_life_file("lifep/GUN30.LIF") 
```