`go vendor` is now `go mod vendor`. So it should be unified to use `go mod vendor` in error messages.

Change-Id: I9e84b9a4f4500659b183b83040867b12a8d174aa
GitHub-Last-Rev: 8516e246d4e7f70bf21311b4cb8cb571a561815b
GitHub-Pull-Request: golang/go#28347
Reviewed-on: https://go-review.googlesource.com/c/144379Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>