docs: scheduler: Fix outdated parameter of rebalance_domains

According to the function prototype of rebalance_domains(), its first
parameter is *rq* and the document need to be updated.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/20211221031818.23186-1-tangyizhou@huawei.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>