doc/go1.13: add id tag to note about removal of NaCl port in Go 1.14

This was in response to a post-merge review comment in
golang.org/cl/185537

Change-Id: I866b3882c8e83bf1fef60115cff5d1c6a9863f09
Reviewed-on: https://go-review.googlesource.com/c/go/+/186319Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.13.html

@@ -102,7 +102,7 @@ Do not send CLs removing the interior tags from such phrases.
 
 <h2 id="ports">Ports</h2>
 
-<p>
+<p id="nacl">
   Go 1.13 is the last release that will run on Native Client (NaCl).
 </p>