perf data: Document missing --force option

Add the --force option to the man page.
Signed-off-by: Sangwon Hong <qpakzk@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1517831315-31490-1-git-send-email-qpakzk@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-data.txt

@@ -30,6 +30,10 @@ OPTIONS for 'convert'
 -i::
 	Specify input perf data file path.
 
+-f::
+--force::
+	Don't complain, do it.
+
 -v::
 --verbose::
         Be more verbose (show counter open errors, etc).