Fix Seq2SeqTrainingArguments docs
# What does this PR do?
This PR fixes the Seq2SeqTrainingArguments docs. The current docs have a problem in rendering as shown in the image below:

![image](https://user-images.githubusercontent.com/29076344/159262299-a854b114-a67e-4ef6-a90a-91b6eb476cb6.png).

Specifically, the parameters added in the sub-class are now showing up in the parameter list but as a messy paragraph at the end.

I believe the issue is because of the indentation in the docs for Seq2SeqTrainingArguments. If there is any other way this should be addressed, please let me know.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).


## Who can review?
Documentation: @sgugger

## EDIT 1:
This is after fixing the indentation:
![image](https://user-images.githubusercontent.com/29076344/159266247-c27b3e97-e357-4a99-a42c-be59e800add0.png)

_The documentation is not available anymore as the PR was closed or merged._<|||||>The style checks are failing because `style_doc.py` check/fixing will undo the changes I made.

I may be totally off here, but:

1. Does this point to a potential issue in the `style_doc.py`? The `style_docstring` method only takes the current file into consider, and not the fact that some additional docstring is added in the beginning or end?

2. Or, should this be handled in the `add_start_docstrings` decorator? In that case, should we indent things manually always?

I believe the second option sounds better since we almost always add only parameters when we use `add_start_docstrings`?

Would love to get your opinion on this. If needed, I can quickly create a PR which does this?

Wdyt @sgugger?
CC: @patil-suraj <|||||>I'd have to take a closer look, but I think it's actually black undoing your changes, not `style_doc`. This may need some more work on our internal tooling.