softlockup: Documentation/sysctl/kernel.txt: fix softlockup_thresh description

- s/s/seconds/

- s/10 seconds/60 seconds/

- Mention the zero-disables-it feature.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>