Documentation: scheduler: fixed 2 typos in sched-nice-design.rst

This patch fixed 2 spelling errors in the documentation.

Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>
Link: https://lore.kernel.org/r/20210516105955.120651-1-henrybear327@gmail.com


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