Fix NER metric example in Overview notebook
Fix errors in `NER metric example` section in `Overview.ipynb`.

```
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-37-ee559b166e25> in <module>()
----> 1 ner_metric = load_metric('seqeval')
      2 references = [['O', 'O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
      3 predictions =  [['O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
      4 ner_metric.compute(predictions, references)


/usr/local/lib/python3.6/dist-packages/datasets/load.py in prepare_module(path, script_version, download_config, download_mode, dataset, force_local_path, **download_kwargs)
    340     if needs_to_be_installed:
    341         raise ImportError(
--> 342             f"To be able to use this {module_type}, you need to install the following dependencies"
    343             f"{[lib_name for lib_name, lib_path in needs_to_be_installed]} using 'pip install "
    344             f"{' '.join([lib_path for lib_name, lib_path in needs_to_be_installed])}' for instance'"

ImportError: To be able to use this metric, you need to install the following dependencies['seqeval'] using 'pip install seqeval' for instance'
```

```
ValueError                                Traceback (most recent call last)
<ipython-input-39-ee559b166e25> in <module>()
      2 references = [['O', 'O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
      3 predictions =  [['O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
----> 4 ner_metric.compute(predictions, references)

/usr/local/lib/python3.6/dist-packages/datasets/metric.py in compute(self, *args, **kwargs)
    378         """
    379         if args:
--> 380             raise ValueError("Please call `compute` using keyword arguments.")
    381 
    382         predictions = kwargs.pop("predictions", None)

ValueError: Please call `compute` using keyword arguments.
```