Typo in modeling_albert.py for mask_token
## 🐛 Bug

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Model I am using (Bert, XLNet....): Albert

Language I am using the model on (English, Chinese....): English

## To Reproduce

```
tokenizer_class, pretrained_weights = AlbertTokenizer, "albert-base-v1"
tokenizer = tokenizer_class.from_pretrained(pretrained_weights)
print(tokenizer.mask_token)   # [MASK]>
print(tokenizer.mask_token_id)  # 1 (same as  <unk>)
```

I think the typo lies here https://github.com/huggingface/transformers/blob/fbaf05bd92249b6dd961f5f8d60eb0892c541ac8/transformers/tokenization_albert.py#L69


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## Expected behavior

mask_token should be "[MASK]" and mask_token_id should be 4

## Environment

* OS: Windows 10
* Python version: 3.6.9
* PyTorch version:  1.2.0
* PyTorch Transformers version (or branch): 2.2.0

Indeed, good catch, thanks! Fixed on master.