[LongformerForQuestionAnswering] fix qa example in docstring
@patrickvonplaten
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4589?src=pr&el=h1) Report
> Merging [#4589](https://codecov.io/gh/huggingface/transformers/pull/4589?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b86e42e0ac1b59f21f0eccf351d3346bbe3ed4eb&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4589/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4589?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4589   +/-   ##
=======================================
  Coverage   78.09%   78.09%           
=======================================
  Files         123      123           
  Lines       20624    20624           
=======================================
  Hits        16106    16106           
  Misses       4518     4518           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4589?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4589/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `97.40% <ø> (ø)` | |

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<|||||>Thanks for the PR! I actually just uploaded a pretrained question answering model from allen ai and changed the docs accordingly. So I think we don't need this PR anymore ;-). See #4593