docs: fix model sharing file names
Author: borisdayma
Pull request: yes

Documentation references `vocab.txt` while I think it should be `vocab.json`.

Additional notes:
* I also have `training_args.bin` and `merges.txt` which I understand are not needed to be uploaded
* I didn't get `added_tokens.json` but I understand it's not always present

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=h1) Report
> Merging [#5855](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8279471506fab5733dab3e2d3a1542010c976d8a?el=desc) will **decrease** coverage by `2.76%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5855/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5855      +/-   ##
==========================================
- Coverage   79.87%   77.11%   -2.77%     
==========================================
  Files         181      181              
  Lines       35788    35788              
==========================================
- Hits        28587    27597     -990     
- Misses       7201     8191     +990     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `21.62% <0.00%> (-78.38%)` | :arrow_down: |
| [src/transformers/modeling\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yZWZvcm1lci5weQ==) | `16.71% <0.00%> (-77.89%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mdW5uZWwucHk=) | `18.53% <0.00%> (-75.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `24.25% <0.00%> (-73.56%)` | :arrow_down: |
| [src/transformers/activations\_tf.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9uc190Zi5weQ==) | `54.16% <0.00%> (-20.84%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.21% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `83.13% <0.00%> (-0.25%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.48% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.90% <0.00%> (+0.63%)` | :arrow_up: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `87.04% <0.00%> (+1.03%)` | :arrow_up: |
| ... and [5 more](https://codecov.io/gh/huggingface/transformers/pull/5855/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=footer). Last update [8279471...d60525f](https://codecov.io/gh/huggingface/transformers/pull/5855?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


It's more general than vocab.txt or vocab.json so I'll let @sgugger chime in as I think he wrote those lines.

I didn't know that when I wrote it (and the file was vocab.json for me ;-) ). This looks good to me with the suggestion, thanks for clarifying!

Looks good on my side, the only file removed from my example would be `training_args.bin` which makes sense.

Let me know if I need to do any other changes

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.


Oops, sorry this got lost in my notifications. Don't hesitate to ping me if it happens again @borisdayma, will merge as soon as the CI is green.