spec: undo spec date change introduced by prior commit

I accidentally included a modified spec in
https://go-review.googlesource.com/27290 .
Remove that change.

Change-Id: Icb62fe829072860e9eb74865d21e06f30efcfd26
Reviewed-on: https://go-review.googlesource.com/27357Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of August 18, 2016",
+	"Subtitle": "Version of May 31, 2016",
 	"Path": "/ref/spec"
 }-->