Fix formatting in DataCollator docs
Noticed the `x_mask_tokens` functions in the data collator docs look a bit strange with the numbered bullets so this PR removes them:

![Screen Shot 2022-09-30 at 12 06 19 PM](https://user-images.githubusercontent.com/59462357/193339707-326cdf8a-7242-4057-aba9-d813eae25ce6.png)
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19267). All of your documentation changes will be reflected on that endpoint.<|||||>Hmm looks like the bullet points are still there even though we didn't include any in Markdown. I wonder if maybe the `doc-builder` automatically adds them before any list elements? cc @mishig25 <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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