fix several typos in Distil* readme
Hi, feel free to reject if you think these changes aren't necessary. Just saw a couple typos while reading the documentation and wanted to help 😄. The first change looks a bit hard to tell what changed, but it was 'superseeds' to 'supersedes'.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3034?src=pr&el=h1) Report
> Merging [#3034](https://codecov.io/gh/huggingface/transformers/pull/3034?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9df74b8bc42eedc496f7148b9370728054ca3b6a?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3034/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3034?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3034      +/-   ##
==========================================
+ Coverage   77.27%   77.29%   +0.01%     
==========================================
  Files          98       98              
  Lines       16037    16037              
==========================================
+ Hits        12393    12395       +2     
+ Misses       3644     3642       -2
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3034?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3034/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `92.54% <0%> (+0.32%)` | :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/3034?src=pr&el=footer). Last update [9df74b8...1482c03](https://codecov.io/gh/huggingface/transformers/pull/3034?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
