doc/go_spec.html: update date

(addressing comment from review of CL 704737)

Change-Id: I483dea046f664035e79c51729203c9a9ff0cbc59
Reviewed-on: https://go-review.googlesource.com/c/go/+/708299
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Language version go1.25 (Feb 25, 2025)",
+	"Subtitle": "Language version go1.26 (Oct 1, 2025)",
 	"Path": "/ref/spec"
 }-->