[release-branch.go1.9] doc: update 1.9.3 release date
    
    Change-Id: I689ccfb8452a170629425dc97da503b28766c6f9
    Reviewed-on: https://go-review.googlesource.com/89035
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    (cherry picked from commit 7abbaddea4819515e1dad817cc1090e18f655b60)
    Reviewed-on: https://go-review.googlesource.com/89055
    Reviewed-by: Andrew Bonventre <andybons@golang.org>