[Fix doc example] Fix checkpoint name in docstring example
# What does this PR do?

Use real name instead of `_CHECKPOINT_FOR_DOC` in docstring example.

## More context

This line in `modeling_speech_to_text_2.py`
```python
>>> tokenizer = Speech2Text2Tokenizer.from_pretrained(_CHECKPOINT_FOR_DOC)
```

gives the following error:
```python
Traceback (most recent call last):
  File "/tmp/tmpgv1pik63/-789619136090462200.py", line 13, in <module>
    tokenizer = Speech2Text2Tokenizer.from_pretrained(_CHECKPOINT_FOR_DOC)
NameError: name '_CHECKPOINT_FOR_DOC' is not defined
```

because `_CHECKPOINT_FOR_DOC` is not defined if a user copies the code example and try to run it.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @ydshieh,

Could you explain this change in a bit more detail? <|||||>> Hey @ydshieh,
> 
> Could you explain this change in a bit more detail?

A docstring example uses the variable _CHECKPOINT_FOR_DOC as the argument for the method from_pretrained. But for a user who copies the example and tries to run it, an error occurs because _CHECKPOINT_FOR_DOC is not defined. It is only defined inside the model file.

I think the code example should be self-contained, and could run after copy-paste.

(I also updated the PR description)