[docs] Fix wrong newline in the middle of a paragraph
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6573?src=pr&el=h1) Report
> Merging [#6573](https://codecov.io/gh/huggingface/transformers/pull/6573?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/cfa26d2b412ac3494eef06506004ca857c115ad9&el=desc) will **increase** coverage by `0.14%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6573/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6573?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6573      +/-   ##
==========================================
+ Coverage   79.24%   79.38%   +0.14%     
==========================================
  Files         156      156              
  Lines       28130    28130              
==========================================
+ Hits        22292    22332      +40     
+ Misses       5838     5798      -40     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6573?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/6573/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWJhcnQucHk=) | `56.25% <0.00%> (-39.07%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/6573/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `93.76% <0.00%> (+3.52%)` | :arrow_up: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6573/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `99.16% <0.00%> (+32.50%)` | :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/6573?src=pr&el=footer). Last update [cfa26d2...0a20942](https://codecov.io/gh/huggingface/transformers/pull/6573?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks!