watchdog: Fix typo in Kconfig

Correct spelling "overidden" to "overridden" in
drivers/watchdog/Kconfig

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>