[docs] duplicate sentences in `document_question_answering.mdx`
### Description
There are duplicate sentences in `document_question_answering.mdx` from line number 40 to 45.

### Document / Language
`document_question_answering.mdx` / [en](https://huggingface.co/docs/transformers/tasks/document_question_answering)

### Suggestion
![Capture 20230524_1541@2x](https://github.com/huggingface/transformers/assets/46880056/899f6de9-3730-44f7-95eb-32f3f7d34be7)
 should be either:

<table>
<tr>
<td> candidate 1 </td> <td> candidate 2 </td>
</tr>
<tr>
<td>

(...), to predict the positions of the start and end tokens of the answer. (...)


</td>
<td>

(...), in order to predict which token is at the start of the answer and which token is at the end of the answer. (...)

</td>
</tr>
</table>

Please let me know which of the two candidates you would prefer. 

I'd prefer candidate 1 personally. Thanks for reporting and looking forward to your PR with a fix!<|||||>Thanks for your feedback and I agree that candidate 1 is better. I opened PR #23735.