docs: fix typo in isa-debugcon example

docs: fix typo in isa-debugcon example

The <serial> opening tag is paired with the </console> closing tag; that's
a mismatch. The question is then whether to modify the former to
<console>, or the latter to </serial>.

Per section "Relationship between serial ports and consoles", <serial> is
used for emulated (not paravirt) consoles, and it's the type that's
suitable for early debug output (such as from firmware). Thus, change
</console> to </serial>.

Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: Andrea Bolognani <abologna@redhat.com>
Fixes: 654968381df0256c047d2ecd4542ccc90dc57ad0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>