go.dev/_content/blog: fix pkg links

Fixes golang/go#49507.

Change-Id: Ieb0f8ebcd42efebc6b608091b1a4e263ef5d1c67
Reviewed-on: https://go-review.googlesource.com/c/website/+/363054
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Website-Publish: Russ Cox <rsc@golang.org>

go.dev/_content/blog/12years.md

@@ -18,7 +18,7 @@ Another part of that consolidation was
 
 In February, the [Go 1.16 release](/blog/go1.16)
 added [macOS ARM64 support](blog/ports),
-added [a file system interface](/pkg/io/fs) and [embedded files](/pkg/embed),
+added [a file system interface](https://golang.org/pkg/io/fs) and [embedded files](https://golang.org/pkg/embed),
 and [enabled modules by default](/blog/go116-module-changes),
 along with the usual assortment of improvements and optimizations.