doc: add missing "Minor revisions" header for 1.9

Change-Id: Ib042e472e62f48a6afaba1762beaf102a9b99cf5
Reviewed-on: https://go-review.googlesource.com/68290Reviewed-by: Russ Cox <rsc@golang.org>

doc/devel/release.html

@@ -30,6 +30,8 @@ Go 1.9 is a major release of Go.
 Read the <a href="/doc/go1.9">Go 1.9 Release Notes</a> for more information.
 </p>
 
+<h3 id="go1.9.minor">Minor revisions</h3>
+
 <p>
 go1.9.1 (released 2017/10/04) includes two security fixes.
 See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.9.1">Go