perf c2c: Update usage for showing memory events

Since commit b027cc6f

 ("perf c2c: Fix 'perf c2c record -e list' to
show the default events used"), "perf c2c" tool can show the memory
events properly, it's no reason to still suggest user to use the
command "perf mem record -e list" for showing events.

This patch updates the usage for showing memory events with command
"perf c2c record -e list".

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20201011121022.22409-1-leo.yan@linaro.org