Docs: fix imports for annotator metrics

[louisguitton]

Description

While preparing my Argilla meetup talk for next week, I noticed that the examples from the docs on Metrics were failing with this error messsage:

ImportError: cannot import name 'AnnotatorMetric' from 'argilla.client.feedback.metrics' 

So I fixed the docstrings.

Type of change

• Documentation update

How Has This Been Tested

I have run no test. This is a 1-line change.

• sphinx-autobuild (read Developer Documentation for more details)

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/) -> I don't think my change is notable and deserves to be in the Changelog