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.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>