cmd/go: change link in error message from /wiki to /doc.

The /doc link is currently a redirect (CL 334389),
but I plan to update it soon with a more detailed guide.

Updates #36460

Change-Id: I9e4a47ad0c8bcb7361cfa3e5b9d07ad241b13ba6
Reviewed-on: https://go-review.googlesource.com/c/go/+/332573


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>