Histogram example: KeyError: 'Tree'

```
cabinetry templates config_histograms.yml     
```

```
  File "/home/turra/micromamba/envs/mamba-python3.11/lib/python3.11/site-packages/cabinetry/templates/builder.py", line 228, in _position_in_file
    position = sample["Tree"]
               ~~~~~~^^^^^^^^
KeyError: 'Tree'
```

It seems it doesn't understand the inputs are histograms.

cabinetry, version 0.6.0