[release-branch.go1.25] spec: update spec date to match release date

Ths spec date in the 1.25 release branch dates back to Feb 2025
which is rather confusing. Moving it forward to 1.25 release date.

This is a roll forward of CL 709515 which was rolled back by CL 709535.

For golang#75743.
Fixes golang#75777.

Change-Id: I18d7ccfc343aa1f8fba78a896fb69ad6eeb182e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/710215
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: Robert Griesemer

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 go1.25 (Aug 12, 2025)",
 	"Path": "/ref/spec"
 }-->