perf script python: Updated trace_unhandled() signature

Default function signature of trace_unhandled() got changed to include a
field dict, but its documentation, perf-script-python.txt has not been
updated.  Fix it.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Pierre Tardy <tardyp@gmail.com>
Fixes: c0251485

 ("perf scripts python: Give field dict to unhandled callback")
Link: http://lkml.kernel.org/r/20170530111827.21732-6-sj38.park@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>