perf sched: Document missing --force option

Add --force to the man page.
Signed-off-by: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1510842367-11011-4-git-send-email-uneedsihyeon@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-sched.txt

@@ -74,6 +74,10 @@ OPTIONS
 --dump-raw-trace=::
         Display verbose dump of the sched data.
 
+-f::
+--force::
+	Don't complain, do it.
+
 OPTIONS for 'perf sched map'
 ----------------------------