doc: ext2: update description of quota options for ext2

ext2 support user/group disk quota by specifying
usrquota/grpquota option on mount, so fix the
description in the doc properly.

Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>