README.heuristic: rework example code.

Specifically: show the use of tcp_dissect_pdus()
   for a TCP heuristic dissector

Change-Id: I02f184b2c8ef6ed128ef3d0bc59eed759aae54bb
Reviewed-on: https://code.wireshark.org/review/5399
Reviewed-by: Bill Meier <wmeier@newsguy.com>