mkfs.f2fs: change -c option description

Current description confuses users like they can add addtional devices
with one -c option using commas(,) at the same time.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/mkfs/f2fs_format_main.c b/mkfs/f2fs_format_main.c
index d9cdd61..b6785cd 100644
--- a/mkfs/f2fs_format_main.c
+++ b/mkfs/f2fs_format_main.c
@@ -50,7 +50,7 @@
 	MSG(0, "\nUsage: mkfs.f2fs [options] device [sectors]\n");
 	MSG(0, "[options]:\n");
 	MSG(0, "  -b filesystem block size [default:4096]\n");
-	MSG(0, "  -c device1[,device2,...] up to 7 additional devices, except meta device\n");
+	MSG(0, "  -c [device_name] up to 7 additional devices, except meta device\n");
 	MSG(0, "  -d debug level [default:0]\n");
 	MSG(0, "  -e [cold file ext list] e.g. \"mp3,gif,mov\"\n");
 	MSG(0, "  -E [hot file ext list] e.g. \"db\"\n");