perf inject: Document missing options

Add the missing --force option 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/1510842367-11011-1-git-send-email-uneedsihyeon@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>