[Longformer] fix model name in examples
This PR fixes the model identifier of Longformer to the new standard fomat <organisation/model_name>
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=h1) Report
> Merging [#4653](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b5015a2a0f4ea63035a877f5626cb0c3ce97e25d&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4653/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4653   +/-   ##
=======================================
  Coverage   77.19%   77.20%           
=======================================
  Files         128      128           
  Lines       21021    21021           
=======================================
+ Hits        16228    16230    +2     
+ Misses       4793     4791    -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4653/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `96.82% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4653/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.83% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4653/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=footer). Last update [b5015a2...dfce20a](https://codecov.io/gh/huggingface/transformers/pull/4653?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
