update URL to now-available blog post

PiperOrigin-RevId: 706799003
Change-Id: Ic5985efa45619873be7b8bf8e84f4009444a5dcd
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/636816
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
TryBot-Bypass: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Chressie Himpel <chressie@google.com>

README.md

diff --git a/README.md b/README.md
index ac98bc6..68e2965 100644
--- a/README.md
+++ b/README.md
@@ -3,4 +3,4 @@
 The program open2opaque migrates Go code using Go Protobuf from the Open API
 to the Opaque API.
 
-See https://opaque-preview.stapelberg.ch/go.dev/blog/protobuf-opaque for context.
+See https://go.dev/blog/protobuf-opaque for context.