The plural is "PDUs", not "PDU:s".

Change-Id: I4bef734ef17b02e254b742744e2debd3038e98a0
Reviewed-on: https://code.wireshark.org/review/3074
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit bd30abf8701c940fc995317e6ac4aee3be04ad29)
Reviewed-on: https://code.wireshark.org/review/3075