doc: fix formatting for Darwin entry

Updates #23011

Change-Id: I38360501c772ddf7cc4bd1b5d7b0225387ead535
Reviewed-on: https://go-review.googlesource.com/c/151361
Reviewed-by: Tobias Klauser <[email protected]>

Author: ianlancetaylor

doc/go1.12.html

@@ -47,7 +47,9 @@ <h3 id="freebsd">FreeBSD</h3>
   12.0+.
 </p>
 
-<p id="darwin">
+<h3 id="darwin">Darwin</h3>
+
+<p>
 Go 1.12 is the last release that will run on macOS 10.10 Yosemite.
 Go 1.13 will require macOS 10.11 El Capitan or later.
 </p>