Fixed bug to calculate correct xpath_sub_list in MarkupLMTokenizer
Fixed the problem where xpath_sub_list was computed incorrectly due to bug in code of MarkupLM Tokenizers.

# What does this PR do?
This PR fixes the bug with function get_xpath_seq method of MarkupLMTokenizer class inside src.transformers.models.markuplm.tokenization_markuplm module. 
Earlier in line number 304 xpath_sub_list was assigned instance of xpath_tag_list which supposedly made embedding of tags with different subscripts similar. Eg li[0] was similar to li[1].
It also fixes the same bug present in src.transformers.models.markuplm.tokenization_markuplm_fast module


Fixes # (issue)
Fixes the issue with incorrect xpath_sub_list computation inside get_xpath_seq method of MarkupLMTokenizer class inside src.transformers.models.markuplm.tokenization_markuplm module and also fixes the same issue in get_xpath_seq method of MarkupLMTokenizerFast class inside src.transformers.models.markuplm.tokenization_markuplm_fast module.

## Before submitting
- [ No] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [Yes ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ No] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ Yes] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ No] Did you write any new necessary tests?


## Who can review?
 @NielsRogge



_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for fixing!