docs: scheduler: Convert schedutil.txt to ReST

All other scheduler documents have been converted to *.rst. Let's do
the same for schedutil.txt.

Also fixed some typos.

Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Link: https://lore.kernel.org/r/20220312070751.16844-1-tangyizhou@huawei.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>