spec: explicitly state the import path for package unsafe

Nowhere in the spec did we mention the import path for package
unsafe. Now we do.

Fixes #22308.

Change-Id: Ifd42c873188e898c597cdee4284e7a9d234a9282
Reviewed-on: https://go-review.googlesource.com/71373


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>