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>

tools/perf/Documentation/perf-stat.txt

@@ -41,13 +41,13 @@ report::
 
 	- a symbolically formed event like 'pmu/param1=0x3,param2/' where
 	  param1 and param2 are defined as formats for the PMU in
-	  /sys/bus/event_sources/devices/<pmu>/format/*
+	  /sys/bus/event_source/devices/<pmu>/format/*
 
 	- a symbolically formed event like 'pmu/config=M,config1=N,config2=K/'
 	  where M, N, K are numbers (in decimal, hex, octal format).
 	  Acceptable values for each of 'config', 'config1' and 'config2'
 	  parameters are defined by corresponding entries in
-	  /sys/bus/event_sources/devices/<pmu>/format/*
+	  /sys/bus/event_source/devices/<pmu>/format/*
 
 -i::
 --no-inherit::