doc: fix two typos in watchdog-api.txt

This patch changes 'go of' to 'go off' and 'pretimout' to
'pretimeout'.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>