docbook: fix sched source file names in device-drivers book

Fix warning since kernel scheduler functions and kernel-doc
notation were moved to other files.

docproc: lnx-33-rc1/kernel/sched.c: No such file or directory

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>