perf timechart: 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/1510842367-11011-3-git-send-email-uneedsihyeon@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-timechart.txt

@@ -50,7 +50,9 @@ TIMECHART OPTIONS
 -p::
 --process::
         Select the processes to display, by name or PID
-
+-f::
+--force::
+	Don't complain, do it.
 --symfs=<directory>::
         Look for files with symbols relative to this directory.
 -n::