modfile: be more clear about go field format

Makes it more clear about the `go` format in `go.mod`. I've seen people get pretty confused about that error message.

Change-Id: Ia1d3dd4250edc9b8fbd159c93c68498d7e8fa783
GitHub-Last-Rev: 5e24006775ef5f18a6dce70fd82f29bbfbdca55c
GitHub-Pull-Request: golang/mod#2
Reviewed-on: https://go-review.googlesource.com/c/mod/+/226438
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
1 file changed