perf buildid-cache: 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/1510842367-11011-6-git-send-email-uneedsihyeon@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-buildid-cache.txt

@@ -24,6 +24,9 @@ OPTIONS
 -a::
 --add=::
         Add specified file to the cache.
+-f::
+--force::
+	Don't complain, do it.
 -k::
 --kcore::
         Add specified kcore file to the cache. For the current host that is