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

tools/perf/Documentation/perf-mem.txt

@@ -28,6 +28,10 @@ OPTIONS
 <command>...::
 	Any command you can specify in a shell.
 
+-f::
+--force::
+	Don't do ownership validation
+
 -t::
 --type=::
 	Select the memory operation type: load or store (default: load,store)