go/types: use singular in version error messages

go/types: use singular in version error messages

This matches the majority use case as well as types2.

For #54511.

Change-Id: If800afc35db8d32cca8329ed9d6a80b4a15c4878
Reviewed-on: https://go-review.googlesource.com/c/go/+/426477
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>