perf c2c: Update documentation for new display option 'peer'

Since the new display option 'peer' is introduced, this patch is to
update the documentation to reflect it.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Ian Rogers <irogers@google.com>
Reviewed-by: Ali Saidi <alisaidi@amazon.com>