Fix LLM EntityLinker task factory typo

[cdjameson]

Description

Change the factory string for the EntityLinker to "llm_entitylinker" from "llm_entity_linker" so that it works, as per https://github.com/explosion/spacy-llm/blob/main/spacy_llm/tasks/init.py#L33. This appears to be the only place it is documented.

Types of change

Documentation change to reflect the API as currently implemented.

Checklist

• [x] I confirm that I have the right to submit this contribution under the project's MIT license. ~~- [ ] I ran the tests, and all new and existing tests passed.~~
• [x] My changes don't require a change to the documentation, or if they do, I've added all required information.