[release-branch.go1.9] doc/contrib.html: fix outdated link
    
    Change-Id: I6d94a14a781a2fbeb90db35ae3490ce2ac5f8ed1
    Reviewed-on: https://go-review.googlesource.com/83315
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    (cherry picked from commit 2fb9fe48c63ed0b7eee36d0d8c2d494ad70602b0)
    Reviewed-on: https://go-review.googlesource.com/89036
    Reviewed-by: Andrew Bonventre <andybons@golang.org>