[doc] fix anchors
the manual anchors end up being duplicated with automatically added anchors and no longer work.

Examples:
https://huggingface.co/docs/transformers/v4.21.1/en/glossary#input-ids
https://huggingface.co/docs/transformers/v4.21.1/en/glossary#position-ids

I confirmed that this fix works via the auto-generated docs link.

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