token-classification: use is_world_process_zero instead of  is_world_master()
Hi,

I just found some leftovers of the `is_world_master()` function in the token classification example.

As this method has been removed, the following error message is thrown when using the `do_prediction` option:

```bash
Traceback (most recent call last):
  File "run_ner.py", line 394, in <module>
    main()
  File "run_ner.py", line 372, in main
    if trainer.is_world_master():
AttributeError: 'Trainer' object has no attribute 'is_world_master'
```

This PR fixes it and uses the new `is_world_process_zero()` method instead!
/cc @sgugger :hugs: 