Use proto_tree_add_expert_format instead of proto_tree_add_text

Change-Id: Ia017fa9535cbc7570ce6bd442972eedd2143825a
Reviewed-on: https://code.wireshark.org/review/4729
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>