18130-type-alias.md: fix relative links to golang.org
18130-type-alias.md: fix relative links to golang.org

Fixes golang/go#21344

Change-Id: I0fef42742a110c1122b55e55435eb99c5b6307ec
Reviewed-on: https://go-review.googlesource.com/53951
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Avelino <8efd86fb78a56a5145ed7739dcb00c78581c5375@avelino.xxx>

golang/proposal,golang/proposal