sched_ext: Documentation: Fix ops table header reference

sched_ext: Documentation: Fix ops table header reference

The "Where to Look" and "ABI Instability" sections state that the ops
table is defined in include/linux/sched/ext.h. However, struct
sched_ext_ops is actually defined in kernel/sched/ext/internal.h, along
with the SCX_OPS_* flags; include/linux/sched/ext.h holds the core data
structures (struct sched_ext_entity, struct scx_dispatch_q, ...) and the
DSQ constants. Point the ops table references to the correct header.

Signed-off-by: Liang Luo <luoliang@kylinos.cn>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>