cmd/present: Fix HTML to use matching header tag

Close the `<h3>` header tag with a closing `</h3>` tag to fix the HTML
syntax.

Change-Id: Ic86c5f31ec5550d6875aa085eed8da6a75881405
Reviewed-on: https://go-review.googlesource.com/2104
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>