Clean groupby error message
Cleans an error message from `groupby.ops`.
Just need to format this with black:

```
Checking black formatting
would reformat /home/vsts/work/1/s/pandas/core/groupby/ops.py
1 file would be reformatted, 870 files would be left unchanged.
```

Thanks! this is ok, though this is not actually a user facing error message.