Fix GPT2 docstring from #1906
Fixes #1906

Changes the GPT2 Tokenizer's docstrings to correctly explain the reason for `add_prefix_space` parameter
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=h1) Report
> Merging [#1911](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/26db31e0c09a8b5e1ca7a61c454b159eab9d86be?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1911/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1911   +/-   ##
=======================================
  Coverage   84.04%   84.04%           
=======================================
  Files          97       97           
  Lines       14333    14333           
=======================================
  Hits        12046    12046           
  Misses       2287     2287
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/1911/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rva2VuaXphdGlvbl9ncHQyLnB5) | `96.72% <ø> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=footer). Last update [26db31e...65c5080](https://codecov.io/gh/huggingface/transformers/pull/1911?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks!