godoc/redirect: make /issue and /issues identical
godoc/redirect: make /issue and /issues identical

I frequently type one when the other is required.
Make them interchangeable.

Change-Id: Ib98d2b67aa42d79e6a8e9fd29fdea17048378eb6
Reviewed-on: https://go-review.googlesource.com/12509
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/tools,argandas/tools,argandas/tools,syreclabs/go-tools,magiccao/tools,jdhenke/tools,nicot/tools,cw2018/tools,springning/tools,cmars/tools,springning/tools,golang/tools,springning/tools,cmars/tools,golang/tools,matryer/tools,matryer/tools,syreclabs/go-tools,argandas/tools,magiccao/tools,nicot/tools,matryer/tools,glasser/golang-tools,syreclabs/go-tools,josharian/tools,josharian/tools,cw2018/tools,jdhenke/tools,glasser/golang-tools,golang/tools,argandas/tools,jdhenke/tools,runcom/tools,cw2018/tools,cmars/tools,runcom/tools,josharian/tools,springning/tools,cw2018/tools,magiccao/tools,glasser/golang-tools,magiccao/tools,jdhenke/tools,syreclabs/go-tools,nicot/tools,runcom/tools,josharian/tools,syreclabs/go-tools,glasser/golang-tools,cmars/tools,matryer/tools,golang/tools,runcom/tools,syreclabs/go-tools,nicot/tools