docs/pcie.txt: Replace ioh3420 with pcie-root-port

docs/pcie.txt: Replace ioh3420 with pcie-root-port

Do not mention ioh3420 in the "how to" doc.
The device still works and can be used by already
existing setups, but no need to be mentioned.

Suggested-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230123174205.683979-1-berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>