sched: Remove USER_SCHED from documentation

USER_SCHED has been removed, so update the documentation
accordingly.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Serge E. Hallyn <serue@us.ibm.com>
LKML-Reference: <4BA9A07E.8070508@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>