perf timechart: Document missing --force option

Add --force to the man page.

Signed-off-by: Sihyeon Jang <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-timechart.txt

···
 -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::