perf report: Fix description for --mem-mode

The "mem-loads" event only works when PEBS is enabled, so add the "/p"
("precise") suffix to the examples.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
LPU-Reference: 20180209163909.9240-1-andi@firstfloor.org
Link: https://lkml.kernel.org/n/tip-v0gcd4u9tktrvjjsp6y7ouv4@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -368,7 +368,7 @@ OPTIONS
 	Use the data addresses of samples in addition to instruction addresses
 	to build the histograms.  To generate meaningful output, the perf.data
 	file must have been obtained using perf record -d -W and using a
-	special event -e cpu/mem-loads/ or -e cpu/mem-stores/. See
+	special event -e cpu/mem-loads/p or -e cpu/mem-stores/p. See
 	'perf mem' for simpler access.
 
 --percent-limit::