perf report: Clarify in help that --children is default

Refer to --no-children, which is what most people probably want.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
LPU-Reference: 20200103183643.149150-1-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>