Fix a typo: Unidirected -> Undirected

As reported in https://ask.wireshark.org/questions/37462/possible-mistake-in-bluetooth-low-energy-hci-event-type

Change-Id: Ia6217531208b00c52c89bcb279cbbe67b5163e96
Reviewed-on: https://code.wireshark.org/review/5004
Reviewed-by: Michael Mann <mmann78@netscape.net>