Fix documentation code to import facebook/detr-resnet-50 model
# What does this PR do?

Changes example import line 

`>>> model = DetrForObjectDetection.from_pretrained("facebook/resnet-50")` 

to 

`>>> model = DetrForObjectDetection.from_pretrained("facebook/detr-resnet-50")` 

As trying to import `"facebook/resnet-50"` raises:

```
OSError: facebook/resnet-50 is not a local folder and is not a valid model identifier listed on 'https://huggingface.co/models'
If this is a private repository, make sure to pass a token having permission to this repo with `use_auth_token` or log in with `huggingface-cli login` and pass `use_auth_token=True`.
```

_The documentation is not available anymore as the PR was closed or merged._