[release-branch.go1.11] cmd/go: don't mention -mod=release

The -mod=release flag is not supported, so this appears to be a
documentation mistake.

Updates #27354.
Fixes #27398.

Change-Id: I895e8d5b4918adcb1f605361773173f312fa7b65
GitHub-Last-Rev: 42bfe0c11e38c90e76887771654ea81af98d50ec
GitHub-Pull-Request: golang/go#27358
Reviewed-on: https://go-review.googlesource.com/132116


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 014901c5)
Reviewed-on: https://go-review.googlesource.com/c/139421


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>