docs: correct number of layers for various xlm models
Hi,

during some NER experiments I found out, that the number of reported layers in the documentation is different compared to the model configuration for some XLM models.

This PR fixes the documentation :)
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=h1) Report
> Merging [#984](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/7729ef738161a0a182b172fcb7c351f6d2b9c50d?src=pr&el=desc) will **increase** coverage by `0.06%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
+ Coverage   79.16%   79.22%   +0.06%     
==========================================
  Files          38       38              
  Lines        6406     6406              
==========================================
+ Hits         5071     5075       +4     
+ Misses       1335     1331       -4
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvZmlsZV91dGlscy5weQ==) | `74.1% <0%> (+2.87%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=footer). Last update [7729ef7...39f51cd](https://codecov.io/gh/huggingface/pytorch-transformers/pull/984?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks Stefan, there were a few other typos in these models details indeed so I'll take care of this in another PR.