[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 014901c5bab2f99af3b1019d5776fa5da6f5bef7)
    Reviewed-on: https://go-review.googlesource.com/c/139421
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>