_content/blog/loopvar-preview: fix Let's Encrypt link

Change-Id: I2b11c98f7ba064752a0a388e81e0a4b7e1be7a3b
Reviewed-on: https://go-review.googlesource.com/c/website/+/529478
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
diff --git a/_content/blog/loopvar-preview.md b/_content/blog/loopvar-preview.md
index f73643c..535f4e8 100644
--- a/_content/blog/loopvar-preview.md
+++ b/_content/blog/loopvar-preview.md
@@ -61,7 +61,7 @@
 
 This kind of mistake has caused production problems at many companies,
 including a
-[publicly documented issue at Lets Encrypt](https://go.googlesource.com/proposal/+/master/design/60078-loopvar.md).
+[publicly documented issue at Lets Encrypt](https://bugzilla.mozilla.org/show_bug.cgi?id=1619047).
 In that instance, the accidental capture of the loop variable was spread across
 multiple functions and much more difficult to notice: