cmd/go: fix 'go help go.mod' example

Dropped the example referred to in the text
when copying this text out of 'go help mod fix'.

Fixes #27083.

Change-Id: I63dfa3033fa2b2408019eef9d8b5a055aa803c57
Reviewed-on: https://go-review.googlesource.com/130140


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>