perf report: Fix -z option

Fix a copy and paste error, -z was setting the group option.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: a.p.zijlstra@chello.nl
Cc: paulus@samba.org
LKML-Reference: <20090630230140.714204656@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>