Docs: Update plugin-missing error message

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Updated the error message displayed when a plugin can't be found. We are getting a lot of people stopping by the chat or filing issues for this particular type of error, and it's typically because they're using something like plugin:foo in the extends array instead of plugin:foo/bar. I've updated the error message to mention this case to hopefully save us some customer support time.

Is there anything you'd like reviewers to focus on?

Does the error message make sense?

[mdjermanovic]

This should be fixed in @eslint/eslintrc:

eslint/eslintrc#12

[nzakas]

Ah forgot about that. Thanks!