fix the mlm longformer example by changing [MASK] to <mask>
This PR fixes an official Huggingface example in the Longformer model

## Before submitting
- [x ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

@Rocketknight1 this PR is to address https://github.com/huggingface/transformers/issues/11488#issuecomment-830065245

Is this what you meant? 


Is there any way to look at the "new" docs locally? Because I don't know how to test my change if it's docs related<|||||>Yes! You can run `make docs` at the root of your clone. You can also run `make html` in the `docs` folder :)<|||||>You'll need to do `pip install .[docs]` beforehand to have all the appropriate dependencies.<|||||>thanks! It's looking good: 
![image](https://user-images.githubusercontent.com/11276933/116860790-61767f80-ac02-11eb-8a53-5533669951a8.png)
<|||||>Great job, thanks for this! I made one minor change - I ran the `black` code formatter to make sure the code fits our style guidelines. You can do this yourself in future by running `pip install -U -e .[quality]` in the project root to install the code formatters, and then `make style` to reformat the code, but it's a very minor detail.

Once the tests check out I'll merge it. Thanks again for making my job as a maintainer easier!<|||||>No problem, this is what open source is about :D