doc/go_mem: update revision date

CL 381315 added major revisions but neglected to update the date.

For #50859.

Change-Id: I086a55f0c80579c479bca5268109c9f3ae680adf
Reviewed-on: https://go-review.googlesource.com/c/go/+/410675
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

doc/go_mem.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Memory Model",
-	"Subtitle": "Version of May 31, 2014",
+	"Subtitle": "Version of June 6, 2022",
 	"Path": "/ref/mem"
 }-->