perf: Rename perf_event_hw_event in design document

perf_event_hw_event has been renamed to perf_event_attr. The
design document was still using the old name, though.

Signed-off-by: Tim Blechmann <tim@klingt.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4B37646A.90108@klingt.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>