doc: change anchor URLs from golang.org/dl/ to /dl/

doc: change anchor URLs from golang.org/dl/ to /dl/

When running locally, it will redirect properly to golang.org/dl/
(see https://github.com/golang/tools/blob/master/cmd/godoc/dl.go).

This is to support domains in prod other than golang.org.

Change-Id: I6d3051fcd7e06a86442324a64d781d8ad95c624f
Reviewed-on: https://go-review.googlesource.com/88679
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>