perf script python: Updated trace_unhandled() signature

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 <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Pierre Tardy <[email protected]>
Fixes: c02514850d67 ("perf scripts python: Give field dict to unhandled callback")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>