Fixing docs for Encoder Decoder Config
This is a small contribution to the EncoderDecoderConfig documentation.
While trying to use this class, I noticed the documentation said that we could pass 2 "encoder" parameters to the class constructor. 
I am almost 99.99% certain it meant we could pass an encoder and a decoder parameter to the class constructor.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5171?src=pr&el=h1) Report
> Merging [#5171](https://codecov.io/gh/huggingface/transformers/pull/5171?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/bc3a0c06075050d3de586c543e4ad6a7efc9260e&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/5171/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5171?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5171   +/-   ##
=======================================
  Coverage   78.31%   78.31%           
=======================================
  Files         137      137           
  Lines       23475    23475           
=======================================
+ Hits        18384    18385    +1     
+ Misses       5091     5090    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5171?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_encoder\_decoder.py](https://codecov.io/gh/huggingface/transformers/pull/5171/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VuY29kZXJfZGVjb2Rlci5weQ==) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5171/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.16% <0.00%> (-0.13%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5171/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.11% <0.00%> (+0.29%)` | :arrow_up: |

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<|||||>Oh, you are 100% correct here @mikaelsouza . This must be pretty confusing in the docs. Thanks for the fix!