doc: fix spec links in Go 1.13 release notes
doc: fix spec links in Go 1.13 release notes

When reading tip.golang.org/doc/go1.13.html, the spec links in
the "Changes to the language" section should point to the updated
spec, not the old one.

Change-Id: I6b13ca0b4c722ed52b84a12a680bece876a4e478
Reviewed-on: https://go-review.googlesource.com/c/go/+/184118
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,akutz/go,golang/go,akutz/go,akutz/go,golang/go