doc: make link relative in 1.12 cgo release notes

Change a link in the cgo section of the 1.12 release notes from

  https://golang.org/cmd/cgo ...

to

  /cmd/cgo/ ...

to uniform it with other links on the page, and to ensure correct
target when the page is displayed on tip.golang.org.

Change-Id: I7653a6ea15ce111a60929c7ae7e9fb0dc9515502
Reviewed-on: https://go-review.googlesource.com/c/156858


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>