go/types: fix error message for consistency
go/types: fix error message for consistency

Follow-up on https://golang.org/cl/271706 .
(Missed a review comment.)

Change-Id: Ibff542f43d721600a2452907c0a20941961e793f
Reviewed-on: https://go-review.googlesource.com/c/go/+/271766
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go