content: rewrite reference to plan9.bell-labs.com to 9p.io

Change-Id: I70dd498b16b128a87f9f41473564587a6bc7d9a6
Reviewed-on: https://go-review.googlesource.com/c/blog/+/192837
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/content/gopher.article b/content/gopher.article
index a5d8ffd..5bb4339 100644
--- a/content/gopher.article
+++ b/content/gopher.article
@@ -20,7 +20,7 @@
 
 .image gopher/avatars.png
 
-Another Bell Labs activity led to Renee creating [[http://plan9.bell-labs.com/plan9/glenda.html][Glenda]], the Plan 9 mascot, a distant cousin of the WFMU gopher.
+Another Bell Labs activity led to Renee creating [[https://9p.io/plan9/glenda.html][Glenda]], the Plan 9 mascot, a distant cousin of the WFMU gopher.
 
 .image gopher/glenda.png