doc/play: update URL for concurrent pi

The old link died; replace with an archive.org copy.

Fixes #13345.

Change-Id: Ic4a7fdcf258e1ff3b4a02ecb4f237ae7db2686c7
Reviewed-on: https://go-review.googlesource.com/18335Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/play/pi.go

 // Concurrent computation of pi.
-// See http://goo.gl/ZuTZM.
+// See https://goo.gl/la6Kli.
 //
 // This demonstrates Go's ability to handle
 // large numbers of concurrent processes.