fixes some key names for in LayoutLMv2 / LayoutXLM tokenizers


# What does this PR do?

in case of left padding_side there was a (probably copy/paste) error assigning the bbox data to the labels.
I consider this to be a typo fix.


## 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?

I guess @sgugger as you reviewed https://github.com/huggingface/transformers/pull/12604
cc @NielsRogge 
