perf mem: Document a missing option

Add the missing --force option on the man page.

Signed-off-by: Sangwon Hong <qpakzk@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1518381517-30766-2-git-send-email-qpakzk@gmail.com


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