'function' object has no attribute 'run'

## Summary
Getting an exception when running the example in `README`:

```
import replicate

export REPLICATE_API_TOKEN=<your token>

replicate.run(
    "stability-ai/stable-diffusion:27b93a2413e7f36cd83da926f3656280b2931564ff050bf9575f1fdf9bcd7478",
    input=prompt
)
```

## Expected behaviour
The model should return an Array containing a URL.