perf list: Fix metricgroups title message

perf list: Fix metricgroups title message

  $ perf list metricgroups

gives

  List of pre-defined events (to be used in -e):

  Metric Groups:

  Backend
  Bad
  BadSpec

But that's incorrect of course because metric groups or metrics can only
be specified with -M. So fix the message to say -e or -M

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221004192634.998984-1-ak@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>