cmd/go: add newline after module-requires-version message

Fixes #30263

Change-Id: Iefb3d8baf815c19eaf915a59048e1da799ca0cdf
Reviewed-on: https://go-review.googlesource.com/c/162957


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>