zonefs: fix documentation typos etc.

Fix typos, spellos, etc. in zonefs.txt.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>