doc/go1.19: use matching closing tag in unix build constraint heading

Change-Id: Idb990eac60e334a5901b2d6cdc2380225d011dd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/412294
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

doc/go1.19.html

@@ -72,7 +72,7 @@ <h3 id="go-doc">Doc Comments</h3>
 as well as support for rendering them to HTML, Markdown, and text.
 </p>
 
-<h3 id="go-unix">New <code>unix</code> build constraint</h4>
+<h3 id="go-unix">New <code>unix</code> build constraint</h3>
 
 <p><!-- CL 389934 --><!-- https://go.dev/issue/20322 --><!-- https://go.dev/issue/51572 -->
   The build constraint <code>unix</code> is now recognized