Typo in warning message
`T5Tokenizer` instead of `XLNetTokenizer`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3219?src=pr&el=h1) Report
> Merging [#3219](https://codecov.io/gh/huggingface/transformers/pull/3219?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d6de6423baf02a971d38ee69824104a1f0f85ad2&el=desc) will **not change** coverage by `%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3219/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3219?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3219   +/-   ##
=======================================
  Coverage   78.14%   78.14%           
=======================================
  Files          98       98           
  Lines       16668    16668           
=======================================
  Hits        13026    13026           
  Misses       3642     3642           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3219?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/3219/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdDUucHk=) | `95.83% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3219/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `93.54% <0.00%> (-0.20%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3219/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.56% <0.00%> (+0.15%)` | :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/3219?src=pr&el=footer). Last update [d6de642...0a77ca6](https://codecov.io/gh/huggingface/transformers/pull/3219?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
