Fix error message for dataclasses.field with positional argument (#11180)
Fixes #10248
The crash due to dataclasses.field was fixed in the recent PR #11137, but the error
message was wrong when positional arguments are used.
I update the error message based on
https://github.com/python/mypy/issues/10248#issuecomment-924904033
(Thanks @JelleZijlstra!)