perf stat: Fix path to PMU formats in documentation

perf stat: Fix path to PMU formats in documentation

As defined in tools/perf/util/pmu.c, the EVENT_SOURCE_DEVICE_PATH is
/sys/bus/event_source/devices/ (no traling 's' in event_source)

This patch corrects the path in the perf stat documentation

Signed-off-by: Jack Henschel <jackdev@mailbox.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jack Henschel <jackdev@mailbox.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: trivial@kernel.org
Link: http://lkml.kernel.org/r/20170824132022.10934-1-jackdev@mailbox.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>