Fix table formatting in SegFormer docs
# What does this PR do?

This PR fixes the Markdown table on the main SegFormer doc page. It was missing the line below the header that makes it a valid Markdown table.

Fixes #15334 


## 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?

@sgugger, @NielsRogge
_The documentation is not available anymore as the PR was closed or merged._