[release-branch.go1.8] x/tools/godoc: fix redirect to Gerrit

Redirects to /r/NNNN broke due to crbug.com/gerrit/6888.
Alternative URLs are available in the meantime: /NNNN and /c/NNNN.
This change uses the /NNNN format.

Fixes golang/go#21235

Change-Id: Ie30e01bedd7a8277aedd4070b5f82a754521ed03
Reviewed-on: https://go-review.googlesource.com/52150
Reviewed-by: Kevin Burke <kev@inburke.com>
Reviewed-on: https://go-review.googlesource.com/52170
Reviewed-by: Andrew Bonventre <andybons@golang.org>
2 files changed