Fix full name of NodeId namespace index field, was a copy&pase error

Reviewed-on: https://code.wireshark.org/review/5668
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

Conflicts:
	plugins/opcua/opcua_application_layer.c

Change-Id: I7b1eea9965880e35c65f04406ff756d245b568d4
Reviewed-on: https://code.wireshark.org/review/5669
Reviewed-by: Michael Mann <mmann78@netscape.net>