[Fix doc example] FlaxMarianPreTrainedModel
# What does this PR do?

Tiny fix:

change
```
>>> tokenizer = MarianTokenizer.from_pretrained("facebook/marian-large-cnn")
```
to 
```
>>> tokenizer = MarianTokenizer.from_pretrained("Helsinki-NLP/opus-mt-en-de")
```

BTW, `Helsinki-NLP/opus-mt-en-de` has no Flax model checkpoint. The next line

```
 >>> model = FlaxMarianMTModel.from_pretrained("Helsinki-NLP/opus-mt-en-de")
```
will fail I think.

_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh `Helsinki-NLP/opus-mt-en-de` already has a flax checkpoint. cf
https://huggingface.co/Helsinki-NLP/opus-mt-en-de/blob/main/flax_model.msgpack<|||||>> @ydshieh `Helsinki-NLP/opus-mt-en-de` already has a flax checkpoint. cf https://huggingface.co/Helsinki-NLP/opus-mt-en-de/blob/main/flax_model.msgpack

Great. Sorry, Google search leads me to the wrong page (`de-en` instead ...)