Documentation: cpuset: Fix 'cpuset.tasks' -> 'tasks'

This looks like it was accidentally caught up in e21a05cb (doc:
cpuset: Update the cpuset flag file, 2010-02-24).

While I'm touching the line, also fix the posessive "cpusets" ->
"cpuset's".

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>