[release-branch.go1.12] doc: don't use "go tool vet" as an example
    
    Updates #30199
    
    Change-Id: Ib4586e3facb8c0985c8882482d94843b648b9d2f
    Reviewed-on: https://go-review.googlesource.com/c/162257
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    (cherry picked from commit ffd096db2b1cff6399eb1f86e5652564ee8ee362)
    Reviewed-on: https://go-review.googlesource.com/c/162238
    Reviewed-by: Andrew Bonventre <andybons@golang.org>