Fix incorrect type hint for lang in run_summarization.py
Author: JohnGiorgi
Pull request: yes

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Fixes an incorrect type hint for the `lang` argument. It was `str` but it should be `Optional[str]` as its default value is `None`. This was reported as an error by `mypy`:

```bash
examples/pytorch/summarization/run_summarization.py:127: error: Incompatible types in assignment (expression has type "None", variable has type "str")
```


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