doc: make the draft 1.13 release notes expected release date accurate

Change-Id: I17148bb2b8021ed305ed0d230675d51c41736a11
GitHub-Last-Rev: 2998d46d10587a850fb36f5784df23b371244479
GitHub-Pull-Request: golang/go#34004
Reviewed-on: https://go-review.googlesource.com/c/go/+/192797Reviewed-by: Ingo Oeser <nightlyone@googlemail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.13.html

@@ -20,7 +20,7 @@ Do not send CLs removing the interior tags from such phrases.
 <p>
   <strong>
     Go 1.13 is not yet released. These are work-in-progress
-    release notes. Go 1.13 is expected to be released in August 2019.
+    release notes. Go 1.13 is expected to be released in September 2019.
   </strong>
 </p>