Update go 1.13 draft release notes expected release date

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>