[doc] typo fix
s/e.g./i.e./ as what follows is not an example, but a "that is" statement + fix language

@sgugger

Apparently, there is no agreement on the comma: https://www.dailywritingtips.com/comma-after-i-e-and-e-g/, especially after i.e. https://english.stackexchange.com/questions/16172/should-i-always-use-a-comma-after-e-g-or-i-e. 

Therefore it's probably the best to stick to whatever manual of style this projects prefers. And, perhaps, even starting a brief guide for the `transformers` manual of style, where you also include all those stylistic recommendations, such as, not documenting `None` as a default for optional objects, the item in question, etc.

My recommendation is that every time a style change is introduced, to change all docs at once, so that the diverse developers will most likely copy/observe how the text is written and follow suite when they create new docs. It's not hard to do:

```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e "s|e\.g\. |e.g., |g; s|i\.e\. |i.e., |g;" {} \;
```
this one fixes a missing comma.<|||||>Thanks @stas00 !<|||||>I had missed your comment. We can do the perl magic thing (though check me with privately before applying it as we are scheduling a few big changes today and tomorrow in preparation for the v4.0 so you don't want to do this in the middle of one ;-) ).

As for the documentation, we can add it to our [documentation guide](https://github.com/huggingface/transformers/tree/master/docs#writing-documentation---specification) (which already contains the not defaults to `None`).<|||||>> I had missed your comment. We can do the perl magic thing (though check me with privately before applying it as we are scheduling a few big changes today and tomorrow in preparation for the v4.0 so you don't want to do this in the middle of one ;-) ).

You can run it any time you know it's a good time. All credits go to perl ;)
 
> As for the documentation, we can add it to our [documentation guide](https://github.com/huggingface/transformers/tree/master/docs#writing-documentation---specification) (which already contains the not defaults to `None`).

That works. Probably start a "grammar style" section.
