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

Change-Id: I17148bb2b8021ed305ed0d230675d51c41736a11
GitHub-Last-Rev: 2998d46
GitHub-Pull-Request: #34004
Reviewed-on: https://go-review.googlesource.com/c/go/+/192797
Reviewed-by: Ingo Oeser <[email protected]>
Reviewed-by: Andrew Bonventre <[email protected]>

Author: alex

doc/go1.13.html

@@ -20,7 +20,7 @@ <h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.13</h2>
 <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>