sched_ext: use correct function name in pick_task_scx() warning message

pick_next_task_scx() was turned into pick_task_scx() since
commit 753e2836 ("sched_ext: Unify regular and core-sched pick
task paths"). Update the outdated message.

Signed-off-by: Honglei Wang <jameshongleiwang@126.com>
Signed-off-by: Tejun Heo <tj@kernel.org>