erofs-utils: manpage: add missing -C option for big pcluster

Update the manpage as well.

Link: https://lore.kernel.org/r/20210430040345.17120-3-xiang@kernel.org
Reviewed-by: Li Guifu <bluce.lee@aliyun.com>
Signed-off-by: Gao Xiang <xiang@kernel.org>
1 file changed