sched/balancing: Fix a couple of outdated function names in comments

The 'idle_balance()' function hasn't existed for years, and there's no
load_balance_newidle() either - both are sched_balance_newidle() today.

Reported-by: Honglei Wang <jameshongleiwang@126.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/ZfAwNufbiyt/5biu@gmail.com