nilfs-clean: fix build warning at nilfs_clean_usage()

Fix the following gcc warning:

  nilfs-clean.c: In function: 'nilfs_clean_usage':
  nilfs-clean.c:336: warning: unknown conversion type character ']' in
  format

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>