perf report: Fix the output for stdio events list

perf report: Fix the output for stdio events list

Changing the output header for reporting forced groups via --groups
option on non grouped events, like:

  $ perf record -e 'cycles,instructions'
  $ perf report --stdio --group

Before:

  # Samples: 24  of event 'anon group { cycles:u, instructions:u }'

After:

  # Samples: 24  of events 'cycles:u, instructions:u'

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Fixes: ad52b8cb4886 ("perf report: Add support to display group output for non group events")
Link: http://lkml.kernel.org/r/20180307155020.32613-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>